[seabios PATCH 0/2] Allow non-contiguous APIC IDs

[Eduardo Habkost]

Now this is an actual proposal I would like to get included on Seabios.

It's much simpler than the previous RFCs. It simply changes the ACPI and mptable
code to not require APIC IDs to be contiguous, eliminates references to the
CountCPUs variable, using a 256-bit APIC ID bitmap to find out which CPUs are
present on boot.

Eduardo Habkost (2):
  acpi: set I/O APIC ID to 0 by default
  allow CPUs to have non-contiguous Local APIC IDs

src/acpi-dsdt.dsl |    4 +++-
 src/acpi.c        |   11 ++++++-----
 src/config.h      |    2 ++
 src/mptable.c     |    2 +-
 src/smp.c         |   17 +++++++++++++++++
 src/util.h        |    1 +
 6 files changed, 30 insertions(+), 7 deletions(-)

[seabios PATCH 1/2] acpi: set I/O APIC ID to 0 by default

[Eduardo Habkost]

When resetting an I/O APIC, its ID is set to 0, so set it to 0 on the
MADT table too.

Signed-off-by: Eduardo Habkost 
---
 src/acpi.c   |    2 +-
 src/config.h |    2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/acpi.c b/src/acpi.c
index 55e4607..3f55de9 100644
--- a/src/acpi.c
+++ b/src/acpi.c
@@ -335,7 +335,7 @@ build_madt(void)
     struct madt_io_apic *io_apic = (void*)apic;
     io_apic->type = APIC_IO;
     io_apic->length = sizeof(*io_apic);
-    io_apic->io_apic_id = CountCPUs;
+    io_apic->io_apic_id = BUILD_IOAPIC_ID;
     io_apic->address = cpu_to_le32(BUILD_IOAPIC_ADDR);
     io_apic->interrupt = cpu_to_le32(0);

diff --git a/src/config.h b/src/config.h
index 3a70867..878c691 100644
--- a/src/config.h
+++ b/src/config.h
@@ -52,9 +52,11 @@
 #define BUILD_PCIMEM64_END        0x10000000000ULL

#define BUILD_IOAPIC_ADDR         0xfec00000
+#define BUILD_IOAPIC_ID           0
 #define BUILD_HPET_ADDRESS        0xfed00000
 #define BUILD_APIC_ADDR           0xfee00000

+
 // Important real-mode segments
 #define SEG_IVT      0x0000
 #define SEG_BDA      0x0040

[seabios PATCH 1/2] acpi: set I/O APIC ID to 0 by default

[Gleb Natapov]

On Thu, Jul 19, 2012 at 05:52:41PM -0300, Eduardo Habkost wrote:
> When resetting an I/O APIC, its ID is set to 0, so set it to 0 on the
> MADT table too.

Actually BIOS needs to configure ioapic id to a uniqe value. This does
not really matter for KVM though.
> Signed-off-by: Eduardo Habkost
>
> src/acpi.c   |    2 +-
>  src/config.h |    2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/src/acpi.c b/src/acpi.c
> index 55e4607..3f55de9 100644
> --- a/src/acpi.c
> +++ b/src/acpi.c
> @@ -335,7 +335,7 @@ build_madt(void)
>      struct madt_io_apic *io_apic = (void*)apic;
>      io_apic->type = APIC_IO;
>      io_apic->length = sizeof(*io_apic);
> io_apic->io_apic_id = CountCPUs;
> io_apic->io_apic_id = BUILD_IOAPIC_ID;
> io_apic->address = cpu_to_le32(BUILD_IOAPIC_ADDR);
> io_apic->interrupt = cpu_to_le32(0);
>
> diff --git a/src/config.h b/src/config.h
> index 3a70867..878c691 100644
> --- a/src/config.h
> +++ b/src/config.h
> @@ -52,9 +52,11 @@
>  #define BUILD_PCIMEM64_END        0x10000000000ULL
>
> #define BUILD_IOAPIC_ADDR         0xfec00000
> +#define BUILD_IOAPIC_ID           0
>  #define BUILD_HPET_ADDRESS        0xfed00000
>  #define BUILD_APIC_ADDR           0xfee00000
>
> // Important real-mode segments
>  #define SEG_IVT      0x0000
> #define SEG_BDA      0x0040
>
> 1.7.10.4

[seabios PATCH 1/2] acpi: set I/O APIC ID to 0 by default

[Eduardo Habkost]

On Fri, Jul 20, 2012 at 12:18:59AM +0300, Gleb Natapov wrote:
> On Thu, Jul 19, 2012 at 05:52:41PM -0300, Eduardo Habkost wrote:
> > When resetting an I/O APIC, its ID is set to 0, so set it to 0 on the
> > MADT table too.
>
> Actually BIOS needs to configure ioapic id to a uniqe value. This does
> not really matter for KVM though.

Where does this requirement comes from? I am guessing it matters only
when the I/O APIC is directly connected to the APIC bus (according to
Intel SDM, that's the case only for old Pentium and P6 CPUs)[1].

Anyway, even if some hardware has this unique-ID requirement, today
Seabios does not fulfill it, leaving the I/O APIC ID as 0. The patch at
least makes the MADT table match reality.

[1] I have checked 3 different machines, and all machines I have looked
have an I/O APIC ID that conflicts with an existing Local APIC ID, on
the ACPI MADT table.

Some iasl dumps may be found online by googling for:
 "Subtable Type : 01" "I/O Apic ID"

I looked at 5 or 6 matches, and almost every one have an I/O APIC ID
conflicting with a Local APIC ID.
> > Signed-off-by: Eduardo Habkost
> >
> > src/acpi.c   |    2 +-
> >  src/config.h |    2 ++
> >  2 files changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/src/acpi.c b/src/acpi.c
> > index 55e4607..3f55de9 100644
> > --- a/src/acpi.c
> > +++ b/src/acpi.c
> > @@ -335,7 +335,7 @@ build_madt(void)
> >      struct madt_io_apic *io_apic = (void*)apic;
> >      io_apic->type = APIC_IO;
> >      io_apic->length = sizeof(*io_apic);
> > io_apic->io_apic_id = CountCPUs;
> > io_apic->io_apic_id = BUILD_IOAPIC_ID;
> > io_apic->address = cpu_to_le32(BUILD_IOAPIC_ADDR);
> > io_apic->interrupt = cpu_to_le32(0);
> >
> > diff --git a/src/config.h b/src/config.h
> > index 3a70867..878c691 100644
> > --- a/src/config.h
> > +++ b/src/config.h
> > @@ -52,9 +52,11 @@
> >  #define BUILD_PCIMEM64_END        0x10000000000ULL
> >
> > #define BUILD_IOAPIC_ADDR         0xfec00000
> > +#define BUILD_IOAPIC_ID           0
> >  #define BUILD_HPET_ADDRESS        0xfed00000
> >  #define BUILD_APIC_ADDR           0xfee00000
> >
> > // Important real-mode segments
> >  #define SEG_IVT      0x0000
> > #define SEG_BDA      0x0040
> >
> > 1.7.10.4

[seabios PATCH 1/2] acpi: set I/O APIC ID to 0 by default

[Gleb Natapov]

On Fri, Jul 20, 2012 at 01:22:43PM -0300, Eduardo Habkost wrote:
> On Fri, Jul 20, 2012 at 12:18:59AM +0300, Gleb Natapov wrote:
> > On Thu, Jul 19, 2012 at 05:52:41PM -0300, Eduardo Habkost wrote:
> > > When resetting an I/O APIC, its ID is set to 0, so set it to 0 on the
> > > MADT table too.
> >
> > Actually BIOS needs to configure ioapic id to a uniqe value. This does
> > not really matter for KVM though.
>
> Where does this requirement comes from? I am guessing it matters only
> when the I/O APIC is directly connected to the APIC bus (according to
> Intel SDM, that's the case only for old Pentium and P6 CPUs)[1].

http://www.intel.com/design/chipsets/datashts/29056601.pdf says that it
should be programmed to unique value. I checked 4 machines on 3 of them
this was the case on one (AMD) there are 3 IOAPICs and they all overlap with
APICs.
> Anyway, even if some hardware has this unique-ID requirement, today
> Seabios does not fulfill it, leaving the I/O APIC ID as 0. The patch at
> least makes the MADT table match reality.

The currant code was written when we could only dream supporting 16 cpus
and back than it did correct thing, but now it just broken. QEMU do not
care about IOAPIC ID for sure so I am OK with the patch.
> [1] I have checked 3 different machines, and all machines I have looked
> have an I/O APIC ID that conflicts with an existing Local APIC ID, on
> the ACPI MADT table.
>
> Some iasl dumps may be found online by googling for:
>  "Subtable Type : 01" "I/O Apic ID"
>
> I looked at 5 or 6 matches, and almost every one have an I/O APIC ID
> conflicting with a Local APIC ID.

1 from 4 for me :)
> > > Signed-off-by: Eduardo Habkost
> > >
> > > src/acpi.c   |    2 +-
> > >  src/config.h |    2 ++
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/src/acpi.c b/src/acpi.c
> > > index 55e4607..3f55de9 100644
> > > --- a/src/acpi.c
> > > +++ b/src/acpi.c
> > > @@ -335,7 +335,7 @@ build_madt(void)
> > >      struct madt_io_apic *io_apic = (void*)apic;
> > >      io_apic->type = APIC_IO;
> > >      io_apic->length = sizeof(*io_apic);
> > > io_apic->io_apic_id = CountCPUs;
> > > io_apic->io_apic_id = BUILD_IOAPIC_ID;
> > > io_apic->address = cpu_to_le32(BUILD_IOAPIC_ADDR);
> > > io_apic->interrupt = cpu_to_le32(0);
> > >
> > > diff --git a/src/config.h b/src/config.h
> > > index 3a70867..878c691 100644
> > > --- a/src/config.h
> > > +++ b/src/config.h
> > > @@ -52,9 +52,11 @@
> > >  #define BUILD_PCIMEM64_END        0x10000000000ULL
> > >
> > > #define BUILD_IOAPIC_ADDR         0xfec00000
> > > +#define BUILD_IOAPIC_ID           0
> > >  #define BUILD_HPET_ADDRESS        0xfed00000
> > >  #define BUILD_APIC_ADDR           0xfee00000
> > >
> > > // Important real-mode segments
> > >  #define SEG_IVT      0x0000
> > > #define SEG_BDA      0x0040
> > >
> > > 1.7.10.4

[seabios PATCH 1/2] acpi: set I/O APIC ID to 0 by default

[Eduardo Habkost]

On Mon, Jul 23, 2012 at 02:42:27PM +0300, Gleb Natapov wrote:
> On Fri, Jul 20, 2012 at 01:22:43PM -0300, Eduardo Habkost wrote:
> > On Fri, Jul 20, 2012 at 12:18:59AM +0300, Gleb Natapov wrote:
> > > On Thu, Jul 19, 2012 at 05:52:41PM -0300, Eduardo Habkost wrote:
> > > > When resetting an I/O APIC, its ID is set to 0, so set it to 0 on the
> > > > MADT table too.
> > >
> > > Actually BIOS needs to configure ioapic id to a uniqe value. This does
> > > not really matter for KVM though.
> >
> > Where does this requirement comes from? I am guessing it matters only
> > when the I/O APIC is directly connected to the APIC bus (according to
> > Intel SDM, that's the case only for old Pentium and P6 CPUs)[1].
>
> http://www.intel.com/design/chipsets/datashts/29056601.pdf says that it
> should be programmed to unique value. I checked 4 machines on 3 of them
> this was the case on one (AMD) there are 3 IOAPICs and they all overlap with
> APICs.

True, but it says "All APIC devices using the APIC bus". Figure 10-1 on
Intel SDM Volume 3 makes me believe the I/O APIC and Local APICs are not
connected to the same APIC bus since Pentium 4.

Anyway, if anybody wants to patch SeaBIOS to guarantee the I/O APIC ID
not conflict with the Local APICs, it would be welcome. While this is
not done, the ACPI tables need to match reality.

(The MP-Table has to be changed too, I will send a new patch soon.
Thanks for pointing that out).
> > Anyway, even if some hardware has this unique-ID requirement, today
> > Seabios does not fulfill it, leaving the I/O APIC ID as 0. The patch at
> > least makes the MADT table match reality.
>
> The currant code was written when we could only dream supporting 16 cpus
> and back than it did correct thing, but now it just broken. QEMU do not
> care about IOAPIC ID for sure so I am OK with the patch.
> > [1] I have checked 3 different machines, and all machines I have looked
> > have an I/O APIC ID that conflicts with an existing Local APIC ID, on
> > the ACPI MADT table.
> >
> > Some iasl dumps may be found online by googling for:
> >  "Subtable Type : 01" "I/O Apic ID"
> >
> > I looked at 5 or 6 matches, and almost every one have an I/O APIC ID
> > conflicting with a Local APIC ID.
>
> 1 from 4 for me :)
> > > > Signed-off-by: Eduardo Habkost
> > > >
> > > > src/acpi.c   |    2 +-
> > > >  src/config.h |    2 ++
> > > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/src/acpi.c b/src/acpi.c
> > > > index 55e4607..3f55de9 100644
> > > > --- a/src/acpi.c
> > > > +++ b/src/acpi.c
> > > > @@ -335,7 +335,7 @@ build_madt(void)
> > > >      struct madt_io_apic *io_apic = (void*)apic;
> > > >      io_apic->type = APIC_IO;
> > > >      io_apic->length = sizeof(*io_apic);
> > > > io_apic->io_apic_id = CountCPUs;
> > > > io_apic->io_apic_id = BUILD_IOAPIC_ID;
> > > > io_apic->address = cpu_to_le32(BUILD_IOAPIC_ADDR);
> > > > io_apic->interrupt = cpu_to_le32(0);
> > > >
> > > > diff --git a/src/config.h b/src/config.h
> > > > index 3a70867..878c691 100644
> > > > --- a/src/config.h
> > > > +++ b/src/config.h
> > > > @@ -52,9 +52,11 @@
> > > >  #define BUILD_PCIMEM64_END        0x10000000000ULL
> > > >
> > > > #define BUILD_IOAPIC_ADDR         0xfec00000
> > > > +#define BUILD_IOAPIC_ID           0
> > > >  #define BUILD_HPET_ADDRESS        0xfed00000
> > > >  #define BUILD_APIC_ADDR           0xfee00000
> > > >
> > > > // Important real-mode segments
> > > >  #define SEG_IVT      0x0000
> > > > #define SEG_BDA      0x0040
> > > >
> > > > 1.7.10.4

[seabios PATCH 2/2] allow CPUs to have non-contiguous Local APIC IDs

[Eduardo Habkost]

Extract Local APIC IDs directly from the CPUs, and instead of check for
"i < CountCPUs", check if the APIC ID was present on boot, when building
ACPI tables and the MP-Table.

This keeps ACPI Processor ID == APIC ID, but allows the
hardware<->Seabios interface be completely APIC-ID based, and Seabios
may change the way ACPI Processor IDs are chosen in the future.

As we currently Seabios support only xAPIC and not x2APIC, the list of
present-on-boot APIC IDs is a 256-bit bitmap. If one day Seabios starts
to support x2APIC, the data structure used to enumerate the APIC IDs
will have to be changed.

Signed-off-by: Eduardo Habkost 
---
 src/acpi-dsdt.dsl |    4 +++-
 src/acpi.c        |    9 +++++----
 src/mptable.c     |    2 +-
 src/smp.c         |   17 +++++++++++++++++
 src/util.h        |    1 +
 5 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/src/acpi-dsdt.dsl b/src/acpi-dsdt.dsl
index 2060686..72dc7d8 100644
--- a/src/acpi-dsdt.dsl
+++ b/src/acpi-dsdt.dsl
@@ -676,6 +676,7 @@ DefinitionBlock (
         /* Methods called by run-time generated SSDT Processor objects */
         Method (CPMA, 1, NotSerialized) {
             // _MAT method - create an madt apic buffer
+            // Arg0 = Processor ID = Local APIC ID
             // Local0 = CPON flag for this cpu
             Store(DerefOf(Index(CPON, Arg0)), Local0)
             // Local1 = Buffer (in madt apic form) to return
@@ -688,6 +689,7 @@ DefinitionBlock (
         }
         Method (CPST, 1, NotSerialized) {
             // _STA method - return ON status of cpu
+            // Arg0 = Processor ID = Local APIC ID
             // Local0 = CPON flag for this cpu
             Store(DerefOf(Index(CPON, Arg0)), Local0)
             If (Local0) { Return(0xF) } Else { Return(0x0) }
@@ -708,7 +710,7 @@ DefinitionBlock (
             Store (PRS, Local5)
             // Local2 = last read byte from bitmap
             Store (Zero, Local2)
-            // Local0 = cpuid iterator
+            // Local0 = Processor ID / APIC ID iterator
             Store (Zero, Local0)
             While (LLess(Local0, SizeOf(CPON))) {
                 // Local1 = CPON flag for this cpu
diff --git a/src/acpi.c b/src/acpi.c
index 3f55de9..d79a1e9 100644
--- a/src/acpi.c
+++ b/src/acpi.c
@@ -326,7 +326,7 @@ build_madt(void)
         apic->length = sizeof(*apic);
         apic->processor_id = i;
         apic->local_apic_id = i;
-        if (i < CountCPUs)
+        if (apic_id_is_present(apic->local_apic_id))
             apic->flags = cpu_to_le32(1);
         else
             apic->flags = cpu_to_le32(0);
@@ -444,6 +444,7 @@ build_ssdt(void)
     }

// build "Method(NTFY, 2) {If (LEqual(Arg0, 0x00)) {Notify(CP00, Arg1)} ...}"
+    // Arg0 = Processor ID = APIC ID
     *(ssdt_ptr++) = 0x14; // MethodOp
     ssdt_ptr = encodeLen(ssdt_ptr, 2+5+(12*acpi_cpus), 2);
     *(ssdt_ptr++) = 'N';
@@ -476,7 +477,7 @@ build_ssdt(void)
     ssdt_ptr = encodeLen(ssdt_ptr, 2+1+(1*acpi_cpus), 2);
     *(ssdt_ptr++) = acpi_cpus;
     for (i=0; i<acpi_cpus; i++)
-        *(ssdt_ptr++) = (i < CountCPUs) ? 0x01 : 0x00;
+        *(ssdt_ptr++) = (apic_id_is_present(i)) ? 0x01 : 0x00;

// store pci io windows: start, end, length
     // this way we don't have to do the math in the dsdt
@@ -655,10 +656,10 @@ build_srat(void)
         core->proximity_lo = curnode;
         memset(core->proximity_hi, 0, 3);
         core->local_sapic_eid = 0;
-        if (i < CountCPUs)
+        if (apic_id_is_present(i))
             core->flags = cpu_to_le32(1);
         else
-            core->flags = 0;
+            core->flags = cpu_to_le32(0);
         core++;
     }

diff --git a/src/mptable.c b/src/mptable.c
index 103f462..9406f98 100644
--- a/src/mptable.c
+++ b/src/mptable.c
@@ -59,7 +59,7 @@ mptable_init(void)
         cpu->apicid = i;
         cpu->apicver = apic_version;
         /* cpu flags: enabled, bootstrap cpu */
-        cpu->cpuflag = ((i<CountCPUs) ? 0x01 : 0x00) | ((i==0) ? 0x02 : 0x00);
+        cpu->cpuflag = (apic_id_is_present(i) ? 0x01 : 0x00) | ((i==0) ? 0x02 : 0x00);
         cpu->cpusignature = cpuid_signature;
         cpu->featureflag = cpuid_features;
         cpu++;
diff --git a/src/smp.c b/src/smp.c
index 8c077a1..af92e4a 100644
--- a/src/smp.c
+++ b/src/smp.c
@@ -36,6 +36,8 @@ wrmsr_smp(u32 index, u64 val)

u32 CountCPUs VAR16VISIBLE;
 u32 MaxCountCPUs VAR16VISIBLE;
+// 256 bits for the found APIC IDs
+u32 FoundAPICIDs[256/32] VAR16VISIBLE;
 extern void smp_ap_boot_code(void);
 ASM16(
     "  .global smp_ap_boot_code\n"
@@ -59,6 +61,12 @@ ASM16(
     "  jmp 1b\n"
     "2:\n"

+    // get apic ID on EBX, set bit on FoundAPICIDs
+    "  mov $1, %eax\n"
+    "  cpuid\n"
+    "  shrl $24, %ebx\n"
+    "  lock bts %ebx, FoundAPICIDs\n"
+
     // Increment the cpu counter
     "  lock incl CountCPUs\n"

@@ -67,6 +75,11 @@ ASM16(
     "  jmp 1b\n"
     );

+int apic_id_is_present(u8 apic_id)
+{
+    return FoundAPICIDs[apic_id/32] & (1 << (apic_id % 32));
+}
+
 // find and initialize the CPUs by launching a SIPI to them
 void
 smp_probe(void)
@@ -82,6 +95,10 @@ smp_probe(void)
         return;
     }

+    // mark the BSP initial APIC ID as found, too:
+    u8 apic_id = ebx>>24;
+    FoundAPICIDs[apic_id/32] |= (1 << (apic_id % 32));
+
     // Init the counter.
     writel(&CountCPUs, 1);

diff --git a/src/util.h b/src/util.h
index ef8ec7c..89e928c 100644
--- a/src/util.h
+++ b/src/util.h
@@ -355,6 +355,7 @@ extern u32 CountCPUs;
 extern u32 MaxCountCPUs;
 void wrmsr_smp(u32 index, u64 val);
 void smp_probe(void);
+int apic_id_is_present(u8 apic_id);

// coreboot.c
 extern const char *CBvendor, *CBpart;

[seabios PATCH 2/2] allow CPUs to have non-contiguous Local APIC IDs

[Kevin O'Connor]

On Thu, Jul 19, 2012 at 05:52:42PM -0300, Eduardo Habkost wrote:
> Extract Local APIC IDs directly from the CPUs, and instead of check for
> "i < CountCPUs", check if the APIC ID was present on boot, when building
> ACPI tables and the MP-Table.
>
> This keeps ACPI Processor ID == APIC ID, but allows the
> hardware<->Seabios interface be completely APIC-ID based, and Seabios
> may change the way ACPI Processor IDs are chosen in the future.
>
> As we currently Seabios support only xAPIC and not x2APIC, the list of
> present-on-boot APIC IDs is a 256-bit bitmap. If one day Seabios starts
> to support x2APIC, the data structure used to enumerate the APIC IDs
> will have to be changed.

Is this SeaBIOS change dependent on a particular version of QEMU?

[...]
> --- a/src/smp.c
> +++ b/src/smp.c
> @@ -36,6 +36,8 @@ wrmsr_smp(u32 index, u64 val)
>
> u32 CountCPUs VAR16VISIBLE;
>  u32 MaxCountCPUs VAR16VISIBLE;
> +// 256 bits for the found APIC IDs
> +u32 FoundAPICIDs[256/32] VAR16VISIBLE;
>  extern void smp_ap_boot_code(void);
>  ASM16(
>      "  .global smp_ap_boot_code\n"
> @@ -59,6 +61,12 @@ ASM16(
>      "  jmp 1b\n"
>      "2:\n"
> // get apic ID on EBX, set bit on FoundAPICIDs
> "  mov $1, %eax\n"
> "  cpuid\n"
> "  shrl $24, %ebx\n"
> "  lock bts %ebx, FoundAPICIDs\n"

Because of the insanity that is 16bit mode, please use explicit size
suffixes on assembler instructions (ie, "movl" and "btsl").

-Kevin

[seabios PATCH 2/2] allow CPUs to have non-contiguous Local APIC IDs

[Eduardo Habkost]

On Thu, Jul 19, 2012 at 09:12:54PM -0400, Kevin O'Connor wrote:
> On Thu, Jul 19, 2012 at 05:52:42PM -0300, Eduardo Habkost wrote:
> > Extract Local APIC IDs directly from the CPUs, and instead of check for
> > "i < CountCPUs", check if the APIC ID was present on boot, when building
> > ACPI tables and the MP-Table.
> >
> > This keeps ACPI Processor ID == APIC ID, but allows the
> > hardware<->Seabios interface be completely APIC-ID based, and Seabios
> > may change the way ACPI Processor IDs are chosen in the future.
> >
> > As we currently Seabios support only xAPIC and not x2APIC, the list of
> > present-on-boot APIC IDs is a 256-bit bitmap. If one day Seabios starts
> > to support x2APIC, the data structure used to enumerate the APIC IDs
> > will have to be changed.
>
> Is this SeaBIOS change dependent on a particular version of QEMU?

No, it's the opposite: QEMU needs this change first, to be able to fix
some CPU socket/core/thread topology bugs caused by incorrect APIC IDs.

For hardware that has contiguous APIC IDs (that's the case for existing
QEMU versions), the OS-visible behavior and resulting tables should be
exactly the same.
> [...]
> > --- a/src/smp.c
> > +++ b/src/smp.c
> > @@ -36,6 +36,8 @@ wrmsr_smp(u32 index, u64 val)
> >
> > u32 CountCPUs VAR16VISIBLE;
> >  u32 MaxCountCPUs VAR16VISIBLE;
> > +// 256 bits for the found APIC IDs
> > +u32 FoundAPICIDs[256/32] VAR16VISIBLE;
> >  extern void smp_ap_boot_code(void);
> >  ASM16(
> >      "  .global smp_ap_boot_code\n"
> > @@ -59,6 +61,12 @@ ASM16(
> >      "  jmp 1b\n"
> >      "2:\n"
> > // get apic ID on EBX, set bit on FoundAPICIDs
> > "  mov $1, %eax\n"
> > "  cpuid\n"
> > "  shrl $24, %ebx\n"
> > "  lock bts %ebx, FoundAPICIDs\n"
>
> Because of the insanity that is 16bit mode, please use explicit size
> suffixes on assembler instructions (ie, "movl" and "btsl").

I'll do. Thanks for the feedback.