[SeaBIOS] [Qemu-devel][RFC PATCH] i386: Add _PXM method to ACPI CPU objects

Vasilis Liaskovitis vasilis.liaskovitis at profitbricks.com
Thu Nov 7 13:41:59 CET 2013

This patch adds a _PXM method to ACPI CPU objects for the pc machine. The _PXM
value is derived from the passed in guest info, same way as CPU SRAT entries.

The motivation for this patch is a CPU hot-unplug/hot-plug bug observed when
using a 3.11 linux guest kernel on a multi-NUMA node qemu/kvm VM. The linux
guest kernel parses the SRAT CPU entries at boot time and stores them in the
array __apicid_to_node. When a CPU is hot-removed, the linux guest kernel
resets the removed CPU's __apicid_to_node entry to NO_NUMA_NODE (kernel commit
c4c60524). When the removed cpu is hot-added again, the linux kernel looks up
the hot-added cpu object's _PXM method instead of somehow re-discovering the
SRAT entry info. With current qemu/seabios, the _PXM method is not found, and
the CPU is thus hot-plugged in the default NUMA node 0. (The problem does not
show up on initial hotplug of a cpu; the PXM method is still not found in this
case, but the kernel still has the correct proximity value from the CPU's SRAT
entry stored in __apicid_to_node)

ACPI spec mentions that the _PXM method is the correct way to determine
proximity information at hot-add time. So far, qemu/seabios do not provide this
method for CPUs. So regardless of kernel behaviour, it is a good idea to add
this _PXM method. Since ACPI table generation has recently been moved from
seabios to qemu, we do this in qemu.

Note that the above hot-remove/hot-add scenario has been tested on an older
qemu + non-upstreamed patches for cpu hot-removal support, and not on qemu
master (since cpu-del support is still not on master). The only testing done
with qemu/seabios master and this patch, are successful boots of multi-node
linux and windows8 guests.

For the initial discussion on seabios and linux-acpi lists see

Signed-off-by: Vasilis Liaskovitis <vasilis.liaskovitis at profitbricks.com>
Reviewed-by: Thilo Fromm <t-lo at thilo-fromm.de>
 hw/i386/acpi-build.c  |    2 ++
 hw/i386/ssdt-proc.dsl |    2 ++
 2 files changed, 4 insertions(+)

diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index 6cfa044..9373f5e 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -603,6 +603,7 @@ static inline char acpi_get_hex(uint32_t val)
 #define ACPI_PROC_OFFSET_CPUHEX (*ssdt_proc_name - *ssdt_proc_start + 2)
 #define ACPI_PROC_OFFSET_CPUID1 (*ssdt_proc_name - *ssdt_proc_start + 4)
 #define ACPI_PROC_OFFSET_CPUID2 (*ssdt_proc_id - *ssdt_proc_start)
+#define ACPI_PROC_OFFSET_CPUPXM (*ssdt_proc_pxm - *ssdt_proc_start)
 #define ACPI_PROC_SIZEOF (*ssdt_proc_end - *ssdt_proc_start)
 #define ACPI_PROC_AML (ssdp_proc_aml + *ssdt_proc_start)
@@ -724,6 +725,7 @@ build_ssdt(GArray *table_data, GArray *linker,
             proc[ACPI_PROC_OFFSET_CPUHEX+1] = acpi_get_hex(i);
             proc[ACPI_PROC_OFFSET_CPUID1] = i;
             proc[ACPI_PROC_OFFSET_CPUID2] = i;
+            proc[ACPI_PROC_OFFSET_CPUPXM] = guest_info->node_cpu[i];
         /* build this code:
diff --git a/hw/i386/ssdt-proc.dsl b/hw/i386/ssdt-proc.dsl
index 8229bfd..7eef8b2 100644
--- a/hw/i386/ssdt-proc.dsl
+++ b/hw/i386/ssdt-proc.dsl
@@ -47,6 +47,8 @@ DefinitionBlock ("ssdt-proc.aml", "SSDT", 0x01, "BXPC", "BXSSDT", 0x1)
  * also updating the C code.
         Name(_HID, "ACPI0007")
+        ACPI_EXTRACT_NAME_BYTE_CONST ssdt_proc_pxm
+        Name(_PXM, 0xAA)
         External(CPMA, MethodObj)
         External(CPST, MethodObj)
         External(CPEJ, MethodObj)

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