linux-kernel - Re: [PATCH v2 7/7] kvm/x86: rework guest entry logic

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Date:   Thu, 20 Jan 2022 12:20:20 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org
Cc:     aleksandar.qemu.devel@...il.com, alexandru.elisei@....com,
        anup.patel@....com, aou@...s.berkeley.edu, atish.patra@....com,
        borntraeger@...ux.ibm.com, bp@...en8.de, catalin.marinas@....com,
        chenhuacai@...nel.org, dave.hansen@...ux.intel.com,
        frankja@...ux.ibm.com, frederic@...nel.org, gor@...ux.ibm.com,
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        tglx@...utronix.de, tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH v2 7/7] kvm/x86: rework guest entry logic

On 1/19/22 11:58, Mark Rutland wrote:
> 
> 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. When context
> tracking is used a small amount of additional time will be accounted
> towards guests; tick-based accounting is unnaffected as IRQs are
> disabled at this point and not enabled until after the return from the
> guest.
> 
> This also corrects (benign) mis-balanced context tracking accounting
> introduced in commits:
> 
>    ae95f566b3d22ade ("KVM: X86: TSCDEADLINE MSR emulation fastpath")
>    26efe2fd92e50822 ("KVM: VMX: Handle preemption timer fastpath")
> 
> Where KVM can enter a guest multiple times, calling vtime_guest_enter()
> without a corresponding call to vtime_account_guest_exit(), and with
> vtime_account_system() called when vtime_account_guest() should be used.
> As account_system_time() checks PF_VCPU and calls account_guest_time(),
> this doesn't result in any functional problem, but is unnecessarily
> confusing.

Yes, I agree.

Acked-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo