Re: [PATCH v3 10/18] KVM: x86/pmu: Document GLOBAL_STATUS_RESET passthrough erratum

[Yosry Ahmed]

On Thu, Nov 05, 2026 at 06:39:19PM -0700, Zide Chen wrote:
> IA32_PERF_GLOBAL_STATUS_RESET, introduced in PerfMon version 4,
> inherits the MSR address and architecturally defined bits from
> IA32_PERF_GLOBAL_OVF_CTRL, while providing additional bits to clear
> new indicators in IA32_PERF_GLOBAL_STATUS.
> 
> From an emulation perspective, KVM handles it identically to the
> OVF_CTRL MSR: reads return zero, and intercepted writes are guarded
> against pmu->global_status_rsvd. No code changes are needed in KVM
> to emulate this MSR.
> 
> However, to reduce VM exits, KVM passes through this MSR in mediated
> PMU mode when GLOBAL_STATUS is also passed through. Consequently,
> writes to bits that are valid on the host but unvirtualized by KVM
> will not trigger a #GP (unlike when the MSR is intercepted). Except
> for the known limitation where host Intel PT in system mode can
> assert Trace_ToPA_PMI (bit 55), unvirtualized status bits are not set
> while the guest is running, so clearing them has no negative effects.
> 
> Document this divergence in errata.rst.
> 
> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> ---
> v3:
> - New patch prompted by Jim and Yosry's review on unvirtualized
>   status bits.
> ---
>  Documentation/virt/kvm/x86/errata.rst | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/kvm/x86/errata.rst b/Documentation/virt/kvm/x86/errata.rst
> index a9cf0e004651..161ba6d1beb4 100644
> --- a/Documentation/virt/kvm/x86/errata.rst
> +++ b/Documentation/virt/kvm/x86/errata.rst
> @@ -84,4 +84,12 @@ memory.
>  
>  Note, this erratum does not affect AMD CPUs, which fully virtualize CR0.CD in
>  hardware, i.e. put the CPU caches into "no fill" mode when CR0.CD=1, even when
> -running in the guest.
> \ No newline at end of file
> +running in the guest.
> +
> +IA32_PERF_GLOBAL_STATUS_RESET
> +-----------------------------
> +To reduce VM-exits on performance-critical paths, KVM passes through writes
> +to IA32_PERF_GLOBAL_STATUS_RESET (0x390) in mediated PMU mode when the guest's
> +counters match hardware, without injecting #GP on unvirtualized bits.

Might be worth clarifying that this refers to the number of counters
matching, not the values in the counters.

> +Because the MSR is read-as-zero and write-to-clear, this cannot leak
> +host information to the guest.

It can let the guest clear host state tho, but I assume this isn't a
problem because GLOBAL_STATUS is saved/restored on guest PMU load/put.
Might be worth clarifying.

> -- 
> 2.56.0
>