[PATCH] add volatile to code that gets optimized out

[Bruce Rogers]

The gcc 4.5 compiler is optimizing out some code used to bring up auxillary processors. Adding volatile to the auxillary processor jump trampoline memory references gets the bios able to boot with -smp >1 again.

Signed-off-by: Bruce Rogers

smp.c |    6 +++--- 
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/smp.c b/src/smp.c 
index dac95bf..ce7c81c 100644 
--- a/src/smp.c 
+++ b/src/smp.c 
@@ -84,11 +84,11 @@ smp_probe(void) 
     writel(&CountCPUs, 1);

// Setup jump trampoline to counter code. 
-    u64 old = *(u64*)BUILD_AP_BOOT_ADDR; 
+    u64 old = *(volatile u64*)BUILD_AP_BOOT_ADDR; 
     // ljmpw $SEG_BIOS, $(smp_ap_boot_code - BUILD_BIOS_ADDR) 
     u64 new = (0xea | ((u64)SEG_BIOS<<24) 
                | (((u32)smp_ap_boot_code - BUILD_BIOS_ADDR) << 8)); 
-    *(u64*)BUILD_AP_BOOT_ADDR = new; 
+    *(volatile u64*)BUILD_AP_BOOT_ADDR = new;

// enable local APIC 
     u32 val = readl(APIC_SVR); 
@@ -117,7 +117,7 @@ smp_probe(void) 
     }

// Restore memory. 
-    *(u64*)BUILD_AP_BOOT_ADDR = old; 
+    *(volatile u64*)BUILD_AP_BOOT_ADDR = old;

MaxCountCPUs = qemu_cfg_get_max_cpus(); 
     if (!MaxCountCPUs || MaxCountCPUs < CountCPUs)

[Kevin O'Connor]

On Tue, Mar 09, 2010 at 08:36:11PM -0700, Bruce Rogers wrote:
> The gcc 4.5 compiler is optimizing out some code used to bring up
> auxillary processors. Adding volatile to the auxillary processor
> jump trampoline memory references gets the bios able to boot with
> -smp >1 again.

Thanks.  Does the following alternate patch work?

--- a/src/smp.c
+++ b/src/smp.c
@@ -103,6 +103,7 @@ smp_probe(void)
     }

// broadcast SIPI
+    barrier();
     writel(APIC_ICR_LOW, 0x000C4500);
     u32 sipi_vector = BUILD_AP_BOOT_ADDR >> 12;
     writel(APIC_ICR_LOW, 0x000C4600 | sipi_vector);

[PATCH] add volatile to code that gets optimized out

[Bruce Rogers]

Yes, that takes care of the problem as well. 
Bruce
> > > On 3/9/2010 at 10:13 PM, Kevin O'Connor  wrote:

On Tue, Mar 09, 2010 at 08:36:11PM -0700, Bruce Rogers wrote:
> The gcc 4.5 compiler is optimizing out some code used to bring up
> auxillary processors. Adding volatile to the auxillary processor
> jump trampoline memory references gets the bios able to boot with
> -smp >1 again.

Thanks.  Does the following alternate patch work?

--- a/src/smp.c
+++ b/src/smp.c
@@ -103,6 +103,7 @@ smp_probe(void)
     }

// broadcast SIPI
+    barrier();
     writel(APIC_ICR_LOW, 0x000C4500);
     u32 sipi_vector = BUILD_AP_BOOT_ADDR >> 12;
     writel(APIC_ICR_LOW, 0x000C4600 | sipi_vector);

[Kevin O'Connor]

On Wed, Mar 10, 2010 at 08:10:02AM -0700, Bruce Rogers wrote:
> Yes, that takes care of the problem as well. 
> Bruce

Thanks - commit 808939c1.

-Kevin