linux-kernel - Re: [PATCH] KVM: nSVM: prepare guest save area while is_guest

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Date:   Thu, 18 Feb 2021 10:12:42 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org, jroedel@...e.de,
        mlevitsk@...hat.com
Subject: Re: [PATCH] KVM: nSVM: prepare guest save area while is_guest_mode
 is true

On Thu, Feb 18, 2021, Paolo Bonzini wrote:
> On 18/02/21 18:42, Sean Christopherson wrote:
> > > The bug is present since commit 06fc7772690d ("KVM: SVM: Activate nested
> > > state only when guest state is complete", 2010-04-25).  Unfortunately,
> > > it is not clear from the commit message what issue exactly led to the
> > > change back then.  It was probably related to svm_set_cr0 however because
> > > the patch series cover letter[1] mentioned lazy FPU switching.
> > 
> > Aha!  It was indeed related to svm_set_cr0().  Specifically, the next patch,
> > commit 66a562f7e257 ("KVM: SVM: Make lazy FPU switching work with nested svm"),
> > added is_nested() checks in update_cr0_intercept() to merge L1's intercepts with
> > L0's intercepts.
> 
> Yeah, the problem is I don't understand why 06fc7772690d fixed things in 11
> year old KVM instead of breaking them, because effectively this patch is
> reverting it.

11 year old KVM didn't grab a different VMCB when updating the intercepts, it
had already copied/merged L1's stuff to L0's VMCB, and then updated L0's VMCB
regardless of is_nested().

> I don't care _that_ much because so much has changed since then; the world
> switch logic is abstracted better nowadays, and it is easier to review the
> change.  But it is weird, nevertheless.
> 
> Paolo
>