linux-kernel - Re: [PATCH] KVM: x86: Include host suspended time in steal time.

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Message-ID: <CABCjUKAkBDCs6knb34sDxOsXT7JwCS_jcknmu9rainx=eM_4zQ@mail.gmail.com>
Date: Thu, 15 Aug 2024 13:33:06 +0900
From: Suleiman Souhlal <suleiman@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Suleiman Souhlal <ssouhlal@...ebsd.org>, Chao Gao <chao.gao@...el.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Include host suspended time in steal time.

On Thu, Aug 15, 2024 at 12:35 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Aug 14, 2024, Suleiman Souhlal wrote:
>
> > With the proposed approach, the steal time page would get copied to the new
> > host and everything would keep working correctly, with the exception of a
> > possible host suspend happening between when the migration started and when
> > it finishes, not being reflected post-migration.  That seems like a
> > reasonable compromise.
>
> Maybe, but I'm not keen on sweeping this under the rug.  Ignoring issues because
> they'll "never" happen has bitten KVM more than once.
>
> At the absolute bare minimum, the flaw needs to be documented, with a suggested
> workaround provided (do KVM on all vCPUs before migrating after suspend), e.g.
> so that userspace can workaround the issue in the unlikely scenario userspace
> does suspend+resume, saves/restores a VM, *and* cares about steal-time.

I can write a comment in record_steal_time() that describes the
scenario, mention it in
the commit message and add something to the steal time part of
Documentation/virt/kvm/x86/msr.rst.

-- Suleiman