Re: [PATCH v5 28/44] KVM: x86/pmu: Load/save GLOBAL_CTRL via entry/exit fields for mediated PMU

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 06, 2025, Sean Christopherson wrote:
> From: Dapeng Mi <dapeng1.mi@linux.intel.com>
> 
> When running a guest with a mediated PMU, context switch PERF_GLOBAL_CTRL
> via the dedicated VMCS fields for both host and guest.  For the host,
> always zero GLOBAL_CTRL on exit as the guest's state will still be loaded
> in hardware (KVM will context switch the bulk of PMU state outside of the
> inner run loop).  For the guest, use the dedicated fields to atomically
> load and save PERF_GLOBAL_CTRL on all entry/exits.
> 
> Note, VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL was introduced by Sapphire
> Rapids, and is expected to be supported on all CPUs with PMU v4+.  WARN if
> that expectation is not met.  Alternatively, KVM could manually save
> PERF_GLOBAL_CTRL via the MSR save list, but the associated complexity and
> runtime overhead is unjustified given that the feature should always be
> available on relevant CPUs.

This is wrong, PMU v4 has been supported since Skylake.

> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 7ab35ef4a3b1..98f7b45ea391 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -787,7 +787,23 @@ static bool intel_pmu_is_mediated_pmu_supported(struct x86_pmu_capability *host_
>  	 * Require v4+ for MSR_CORE_PERF_GLOBAL_STATUS_SET, and full-width
>  	 * writes so that KVM can precisely load guest counter values.
>  	 */
> -	return host_pmu->version >= 4 && host_perf_cap & PERF_CAP_FW_WRITES;
> +	if (host_pmu->version < 4 || !(host_perf_cap & PERF_CAP_FW_WRITES))
> +		return false;
> +
> +	/*
> +	 * All CPUs that support a mediated PMU are expected to support loading
> +	 * and saving PERF_GLOBAL_CTRL via dedicated VMCS fields.
> +	 */
> +	if (WARN_ON_ONCE(!cpu_has_load_perf_global_ctrl() ||
> +			 !cpu_has_save_perf_global_ctrl()))
> +		return false;

And so this WARN fires due to cpu_has_save_perf_global_ctrl() being false.  The
bad changelog is mine, but the code isn't entirely my fault.  I did suggest the
WARN in v3[1], probably because I forgot when PMU v4 was introduced and no one
corrected me.

v4 of the series[2] then made cpu_has_save_perf_global_ctrl() a hard requirement,
based on my miguided feedback.

   * Only support GLOBAL_CTRL save/restore with VMCS exec_ctrl, drop the MSR
     save/retore list support for GLOBAL_CTRL, thus the support of mediated
     vPMU is constrained to SapphireRapids and later CPUs on Intel side.

Doubly frustrating is that this was discussed in the original RFC, where Jim
pointed out[3] that requiring VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL would prevent
enabling the mediated PMU on Skylake+, and I completely forgot that conversation
by the time v3 of the series rolled around :-(

As mentioned in the discussion with Jim, _if_ PMU v4 was introduced with ICX (or
later), then I'd be in favor of making VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL a hard
requirement.  But losing supporting Skylake+ is a bit much.

There are a few warts with nVMX's use of the auto-store list that need to be
cleaned up, but on the plus side it's also a good excuse to clean up
{add,clear}_atomic_switch_msr(), which have accumulated some cruft and quite a
bit of duplicate code.  And while I still dislike using the auto-store list, the
code isn't as ugly as it was back in v3 because we _can_ make the "load" VMCS
controls mandatory without losing support for any CPUs (they predate PMU v4).

[1] https://lore.kernel.org/all/ZzyWKTMdNi5YjvEM@google.com
[2] https://lore.kernel.org/all/20250324173121.1275209-1-mizhang@google.com
[3] https://lore.kernel.org/all/CALMp9eQ+-wcj8QMmFR07zvxFF22-bWwQgV-PZvD04ruQ=0NBBA@mail.gmail.com

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