linux-kernel - Re: [PATCH 00/18] KVM: x86: Fastpath cleanups and PMU prep work

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Message-ID: <129c59a7-1715-41f4-b839-19e6a0adb9f1@amd.com>
Date: Thu, 7 Aug 2025 11:53:24 +0530
From: Sandipan Das <sandipan.das@....com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, Xin Li
 <xin@...or.com>, Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: Re: [PATCH 00/18] KVM: x86: Fastpath cleanups and PMU prep work

On 06-08-2025 00:35, Sean Christopherson wrote:
> This is a prep series for the mediated PMU, and for Xin's series to add
> support for the immediate forms of RDMSR and WRMSRNS (I'll post a v3 of
> that series on top of this).
> 
> The first half cleans up a variety of warts and flaws in the VM-Exit fastpath
> handlers.  The second half cleans up the PMU code related to "triggering"
> instruction retired and branches retired events.  The end goal of the two
> halves (other than general cleanup) is to be able bail from the fastpath when
> using the mediated PMU and the guest is counting instructions retired, with
> minimal overhead, e.g. without having to acquire SRCU.
> 
> Because the mediated PMU context switches PMU state _outside_ of the fastpath,
> the mediated PMU won't be able to increment PMCs in the fastpath, and so won't
> be able to skip emulated instructions in the fastpath if the vCPU is counting
> instructions retired.
> 
> The last patch to handle INVD in the fastpath is a bit dubious.  It works just
> fine, but it's dangerously close to "just because we can, doesn't mean we
> should" territory.  I added INVD to the fastpath before I realized that
> MSR_IA32_TSC_DEADLINE could be handled in the fastpath irrespective of the
> VMX preemption timer, i.e. on AMD CPUs.  But being able to use INVD to test
> the fastpath is still super convenient, as there are no side effects (unless
> someone ran the test on bare metal :-D), no register constraints, and no
> vCPU model requirements.  So, I kept it, because I couldn't come up with a
> good reason not to.
> 
> Sean Christopherson (18):
>   KVM: SVM: Skip fastpath emulation on VM-Exit if next RIP isn't valid
>   KVM: x86: Add kvm_icr_to_lapic_irq() helper to allow for fastpath IPIs
>   KVM: x86: Only allow "fast" IPIs in fastpath WRMSR(X2APIC_ICR) handler
>   KVM: x86: Drop semi-arbitrary restrictions on IPI type in fastpath
>   KVM: x86: Unconditionally handle MSR_IA32_TSC_DEADLINE in fastpath
>     exits
>   KVM: x86: Acquire SRCU in WRMSR fastpath iff instruction needs to be
>     skipped
>   KVM: x86: Unconditionally grab data from EDX:EAX in WRMSR fastpath
>   KVM: x86: Fold WRMSR fastpath helpers into the main handler
>   KVM: x86/pmu: Move kvm_init_pmu_capability() to pmu.c
>   KVM: x86/pmu: Add wrappers for counting emulated instructions/branches
>   KVM: x86/pmu: Calculate set of to-be-emulated PMCs at time of WRMSRs
>   KVM: x86/pmu: Rename pmc_speculative_in_use() to
>     pmc_is_locally_enabled()
>   KVM: x86/pmu: Open code pmc_event_is_allowed() in its callers
>   KVM: x86/pmu: Drop redundant check on PMC being globally enabled for
>     emulation
>   KVM: x86/pmu: Drop redundant check on PMC being locally enabled for
>     emulation
>   KVM: x86/pmu: Rename check_pmu_event_filter() to
>     pmc_is_event_allowed()
>   KVM: x86: Push acquisition of SRCU in fastpath into
>     kvm_pmu_trigger_event()
>   KVM: x86: Add a fastpath handler for INVD
> 
>  arch/x86/include/asm/kvm_host.h |   3 +
>  arch/x86/kvm/lapic.c            |  59 ++++++++----
>  arch/x86/kvm/lapic.h            |   3 +-
>  arch/x86/kvm/pmu.c              | 155 +++++++++++++++++++++++++-------
>  arch/x86/kvm/pmu.h              |  60 ++-----------
>  arch/x86/kvm/svm/svm.c          |  14 ++-
>  arch/x86/kvm/vmx/nested.c       |   2 +-
>  arch/x86/kvm/vmx/pmu_intel.c    |   2 +-
>  arch/x86/kvm/vmx/vmx.c          |   2 +
>  arch/x86/kvm/x86.c              |  85 +++++-------------
>  arch/x86/kvm/x86.h              |   1 +
>  11 files changed, 218 insertions(+), 168 deletions(-)
> 
> 
> base-commit: 196d9e72c4b0bd68b74a4ec7f52d248f37d0f030

No issues observed with KVM Unit Tests on recent AMD platforms (Milan, Genoa and Turin).