linux-kernel - Re: [PATCH 5/5] kvm/x86: rework guest entry logic

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Message-ID: <YeCQeHbswboaosoV@google.com>
Date:   Thu, 13 Jan 2022 20:50:00 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, aleksandar.qemu.devel@...il.com,
        alexandru.elisei@....com, anup.patel@....com,
        aou@...s.berkeley.edu, atish.patra@....com,
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        catalin.marinas@....com, chenhuacai@...nel.org,
        dave.hansen@...ux.intel.com, david@...hat.com,
        frankja@...ux.ibm.com, frederic@...nel.org, gor@...ux.ibm.com,
        hca@...ux.ibm.com, imbrenda@...ux.ibm.com, james.morse@....com,
        jmattson@...gle.com, joro@...tes.org, kvm@...r.kernel.org,
        maz@...nel.org, mingo@...hat.com, mpe@...erman.id.au,
        nsaenzju@...hat.com, palmer@...belt.com, paulmck@...nel.org,
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        tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH 5/5] kvm/x86: rework guest entry logic

On Tue, Jan 11, 2022, Mark Rutland wrote:
> For consistency and clarity, migrate x86 over to the generic helpers for
> guest timing and lockdep/RCU/tracing management, and remove the
> x86-specific helpers.
> 
> Prior to this patch, the guest timing was entered in
> kvm_guest_enter_irqoff() (called by svm_vcpu_enter_exit() and
> svm_vcpu_enter_exit()), and was exited by the call to
> vtime_account_guest_exit() within vcpu_enter_guest().
> 
> To minimize duplication and to more clearly balance entry and exit, both
> entry and exit of guest timing are placed in vcpu_enter_guest(), using
> the new guest_timing_{enter,exit}_irqoff() helpers. This may result in a
> small amount of additional time being acounted towards guests.

This can be further qualified to state that it only affects time accounting when
using context tracking; tick-based accounting is unaffected because IRQs are
disabled the entire time.

And this might actually be a (benign?) bug fix for context tracking accounting in
the EXIT_FASTPATH_REENTER_GUEST case (commits ae95f566b3d2 "KVM: X86: TSCDEADLINE
MSR emulation fastpath" and 26efe2fd92e5, "KVM: VMX: Handle preemption timer
fastpath").  In those cases, KVM will enter the guest multiple times without
bouncing through vtime_account_guest_exit().  That means vtime_guest_enter() will
be called when the CPU is already "in guest", and call vtime_account_system()
when it really should call vtime_account_guest().  account_system_time() does
check PF_VCPU and redirect to account_guest_time(), so it appears to be benign,
but it's at least odd.

> Other than this, there should be no functional change as a result of
> this patch.

...

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e50e97ac4408..bd3873b90889 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9876,6 +9876,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		set_debugreg(0, 7);
>  	}
>  
> +	guest_timing_enter_irqoff();
> +
>  	for (;;) {
>  		/*
>  		 * Assert that vCPU vs. VM APICv state is consistent.  An APICv
> @@ -9949,7 +9951,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	 * of accounting via context tracking, but the loss of accuracy is
>  	 * acceptable for all known use cases.
>  	 */
> -	vtime_account_guest_exit();
> +	guest_timing_exit_irqoff();
>  
>  	if (lapic_in_kernel(vcpu)) {
>  		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;