linux-kernel - Re: [PATCH v2 0/3] KVM: Properly account for guest CPU time

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Message-ID: <YHXUFJuLXY8VZw3B@google.com>
Date:   Tue, 13 Apr 2021 17:25:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Tokarev <mjt@....msk.ru>
Subject: Re: [PATCH v2 0/3] KVM: Properly account for guest CPU time

On Tue, Apr 13, 2021, Wanpeng Li wrote:
> The bugzilla https://bugzilla.kernel.org/show_bug.cgi?id=209831
> reported that the guest time remains 0 when running a while true
> loop in the guest.
> 
> The commit 87fa7f3e98a131 ("x86/kvm: Move context tracking where it
> belongs") moves guest_exit_irqoff() close to vmexit breaks the
> tick-based time accouting when the ticks that happen after IRQs are
> disabled are incorrectly accounted to the host/system time. This is
> because we exit the guest state too early.
> 
> This patchset splits both context tracking logic and the time accounting 
> logic from guest_enter/exit_irqoff(), keep context tracking around the 
> actual vmentry/exit code, have the virt time specific helpers which 
> can be placed at the proper spots in kvm. In addition, it will not 
> break the world outside of x86.

IMO, this is going in the wrong direction.  Rather than separate context tracking,
vtime accounting, and KVM logic, this further intertwines the three.  E.g. the
context tracking code has even more vtime accounting NATIVE vs. GEN vs. TICK
logic baked into it.

Rather than smush everything into context_tracking.h, I think we can cleanly
split the context tracking and vtime accounting code into separate pieces, which
will in turn allow moving the wrapping logic to linux/kvm_host.h.  Once that is
done, splitting the context tracking and time accounting logic for KVM x86
becomes a KVM detail as opposed to requiring dedicated logic in the context
tracking code.

I have untested code that compiles on x86, I'll send an RFC shortly.

> v1 -> v2:
>  * split context_tracking from guest_enter/exit_irqoff
>  * provide separate vtime accounting functions for consistent
>  * place the virt time specific helpers at the proper splot 
> 
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: Michael Tokarev <mjt@....msk.ru>
> 
> Wanpeng Li (3):
>   context_tracking: Split guest_enter/exit_irqoff
>   context_tracking: Provide separate vtime accounting functions
>   x86/kvm: Fix vtime accounting
> 
>  arch/x86/kvm/svm/svm.c           |  6 ++-
>  arch/x86/kvm/vmx/vmx.c           |  6 ++-
>  arch/x86/kvm/x86.c               |  1 +
>  include/linux/context_tracking.h | 84 +++++++++++++++++++++++++++++++---------
>  4 files changed, 74 insertions(+), 23 deletions(-)
> 
> -- 
> 2.7.4
>