linux-kernel - Re: [PATCH v2 1/4] KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE

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Date:   Thu, 7 Jan 2021 11:12:14 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 1/4] KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES
 on nested vmexit

On Thu, Jan 07, 2021, Paolo Bonzini wrote:
> On 07/01/21 18:00, Sean Christopherson wrote:
> > Ugh, I assume this is due to one of the "premature" nested_ops->check_events()
> > calls that are necessitated by the event mess?  I'm guessing kvm_vcpu_running()
> > is the culprit?
> > 
> > If my assumption is correct, this bug affects nVMX as well.
> 
> Yes, though it may be latent.  For SVM it was until we started allocating
> svm->nested on demand.
> 
> > Rather than clear the request blindly on any nested VM-Exit, what
> > about something like the following?
> 
> I think your patch is overkill, KVM_REQ_GET_NESTED_STATE_PAGES is only set
> from KVM_SET_NESTED_STATE so it cannot happen while the VM runs.

Yeah, which is why I was hoping we could avoid clearing the request on every
nested exit.

> Something like this is small enough and works well.

I've no argument against it working, rather that I dislike clearing the request
on every exit.  Except for the ->check_events() case, hitting the scenario where
there's a pending request at the time of nested VM-Exit would ideally be treated
as a KVM bug.

On the other hand, clearing nested-specific request on nested VM-Exit is
logically sound, so I guess I'm ok with the minimal patch.