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

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Message-ID: <cf1b338c-68bc-6e7e-1a10-98bc653d34ce@redhat.com>
Date:   Thu, 18 Feb 2021 19:00:40 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 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.

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