linux-kernel - Re: [PATCH 8/8] KVM: nSVM: read only changed fields of the nested guest data area

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Date:   Thu, 20 Aug 2020 12:18:28 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 8/8] KVM: nSVM: read only changed fields of the nested
 guest data area

On 20/08/20 12:05, Maxim Levitsky wrote:
>> You probably should set clean to 0 also if the guest doesn't have the
>> VMCBCLEAN feature (so, you first need an extra patch to add the
>> VMCBCLEAN feature to cpufeatures.h).  It's probably best to cache the
>> guest vmcbclean in struct vcpu_svm, too.
> Right, I totally forgot about this one.
> 
> One thing why I made this patch optional, is that I can instead drop it,
> and not 'read back' the saved area on vmexit, this will probably be faster
> that what this optimization does. What do you think? Is this patch worth it?
> (I submitted it because I already implemented this and wanted to hear opinion
> on this).

Yeah, good point.  It's one copy either way, either on vmexit (and
partly on vmentry depending on clean bits) or on vmentry.  I had not
considered the need to copy from vmcb02 to the cached vmcb12 on vmexit. :(

Let's shelve this for a bit, and revisit it once we have separate vmcb01
and vmcb02.  Then we might still use the clean bits to avoid copying
data from vmcb12 to vmcb02, including avoiding consistency checks
because we know the vmcb02 data is legit.

Patches 1-5 are still worthwhile, so you can clean them up and send them.

Paolo