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

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Message-ID: <50bcc6e3-5f6e-32a3-6975-db7d857e88f3@redhat.com>
Date:   Thu, 18 Feb 2021 19:28:55 +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 19:12, Sean Christopherson wrote:
>> 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().

Ok, so the bug was introduced when adding recalc_intercepts, which threw 
away the intercept manipulations that svm_set_cr0 had done in the 
meanwhile.  That's commit 384c63684397 ("KVM: SVM: Add function to 
recalculate intercept masks", 2011-01-12).

That piece of information makes me feel less uneasy about missing 
something.  recalc_intercepts has been there for a long time, but not as 
long as 06fc7772690d.

Paolo