Re: Small question about reserved bits in MSR_AMD64_PERF_CNTR_GLOBAL_STATUS_CLR

[dongli.zhang@oracle.com]

On 9/16/24 11:54 AM, Maxim Levitsky wrote:
> Hi!
> 
> We recently saw a failure in one of the aws VM instances that causes the following error during the guest boot:
> 
>  0.480051] unchecked MSR access error: WRMSR to 0xc0000302 (tried to write 0x040000000000001f) at rIP: 0xffffffff96c093e2 (amd_pmu_cpu_reset.constprop.0+0x42/0x80)
> 
> 
> I investigated the issue and I see that the hypervisor does expose PerfmonV2, but not the LBRv2 support:
> 
> #  cpuid -1 -l 0x80000022 
> CPU:
>    Extended Performance Monitoring and Debugging (0x80000022):
>       AMD performance monitoring V2         = true
>       AMD LBR V2                            = false
>       AMD LBR stack & PMC freezing          = false
>       number of core perf ctrs              = 0x5 (5)
>       number of LBR stack entries           = 0x0 (0)
>       number of avail Northbridge perf ctrs = 0x0 (0)
>       number of available UMC PMCs          = 0x0 (0)
>       active UMCs bitmask                   = 0x0
> 
> I also verified that I can write 0x1f to 0xc0000302 but not 0x040000000000001f:
> 
> # wrmsr 0xc0000302 0x1f
> # wrmsr 0xc0000302 0x040000000000001f
> wrmsr: CPU 0 cannot set MSR 0xc0000302 to 0x040000000000001f
> #
> 
> The AMD's APM is not clear on what should happen if unsupported bits are attempted to be cleared
> using this MSR.
> 
> Also I noticed that amd_pmu_v2_handle_irq writes 0xffffffffffffffff to this msrs.
> It has the following code:
> 
> 
> 	WARN_ON(status > 0);
> 
> 	/* Clear overflow and freeze bits */
> 	amd_pmu_ack_global_status(~status);
> 
> 
> This implies that it is OK to set all bits in this MSR.
> 

To share my data point on QEMU+KVM: I am not able to reproduce with the most
recent QEMU (not AWS) + below patch.

[PATCH v2 2/4] i386/cpu: Add PerfMonV2 feature bit
https://lore.kernel.org/all/69905b486218f8287b9703d1a9001175d04c2f02.1723068946.git.babu.moger@amd.com/

Both my VM and KVM are 6.10.

vm# cpuid -1 -l 0x80000022
CPU:
   Extended Performance Monitoring and Debugging (0x80000022):
      AMD performance monitoring V2         = true
      AMD LBR V2                            = false
      AMD LBR stack & PMC freezing          = false
      number of core perf ctrs              = 0x6 (6)
      number of LBR stack entries           = 0x0 (0)
      number of avail Northbridge perf ctrs = 0x0 (0)
      number of available UMC PMCs          = 0x0 (0)
      active UMCs bitmask                   = 0x0


Both writes are passed.

vm# wrmsr 0xc0000302 0x1f
vm# wrmsr 0xc0000302 0x040000000000001f

Here is bcc output. Both writes are good.

kvm# /usr/share/bcc/tools/trace -t -C 'kvm_pmu_set_msr "%x", retval'
... ...
4.748614 19  43545   43550   CPU 0/KVM       kvm_pmu_set_msr  0
10.97396 19  43545   43550   CPU 0/KVM       kvm_pmu_set_msr  0

Dongli Zhang