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Re: [Xen-devel] [PATCH v2 1/2] x86/VPMU: return correct fixed PMC count

On 11/25/2015 04:55 AM, Jan Beulich wrote:
On 25.11.15 at 00:53, <bgregg@xxxxxxxxxxx> wrote:
--- a/xen/arch/x86/cpu/vpmu_intel.c
+++ b/xen/arch/x86/cpu/vpmu_intel.c
@@ -166,10 +166,10 @@ static int core2_get_arch_pmc_count(void)
  static int core2_get_fixed_pmc_count(void)
-    u32 eax;
+    u32 edx;
- eax = cpuid_eax(0xa);
-    return MASK_EXTR(eax, PMU_FIXED_NR_MASK);
+    edx = cpuid_edx(0xa);
+    return MASK_EXTR(edx, PMU_FIXED_NR_MASK);
/* edx bits 5-12: Bit width of fixed-function performance counters */
I'll commit as is since it's an immediate improvement, but I don't think
this is sufficient: The SDM clearly says "if Version ID > 1", which isn't
being tested here or in the immediately following function.

Are you referring to the statement in 18.2.2:
"The enhanced features provided by architectural performance monitoring version 2 include the following" ?

I'd expect CPUID to report zeroes for those enhanced featured in v1.

However, I just noticed that various control and status registers are not available for v1. I wonder whether we should even support version 1 since we'd need to add whole lot of 'if (supported)' throughout the code plus there are some assumptions about existence of IA32_PERF_GLOBAL_CTRL so we'll need to add additional logic to handle that too. And it's not clear to me if it's all worth it.


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