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Subject: Re: [PATCH v4 10/12] KVM: x86: never write to memory from
 kvm_vcpu_check_block()
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Jim Mattson <jmattson@google.com>
Cc:     Sean Christopherson <seanjc@google.com>, alexandru.elisei@arm.com,
        anup@brainfault.org, aou@eecs.berkeley.edu, atishp@atishpatra.org,
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Date:   Thu, 14 Dec 2023 00:44:12 +0200
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On Wed, 2023-12-13 at 14:31 -0800, Jim Mattson wrote:
> On Wed, Dec 13, 2023 at 2:25 PM Maxim Levitsky <mlevitsk@redhat.com> wrote:
> > On Tue, 2023-12-12 at 07:28 -0800, Sean Christopherson wrote:
> > > On Sun, Dec 10, 2023, Jim Mattson wrote:
> > > > On Thu, Dec 7, 2023 at 8:21 AM Sean Christopherson <seanjc@google.com> wrote:
> > > > > Doh.  We got the less obvious cases and missed the obvious one.
> > > > > 
> > > > > Ugh, and we also missed a related mess in kvm_guest_apic_has_interrupt().  That
> > > > > thing should really be folded into vmx_has_nested_events().
> > > > > 
> > > > > Good gravy.  And vmx_interrupt_blocked() does the wrong thing because that
> > > > > specifically checks if L1 interrupts are blocked.
> > > > > 
> > > > > Compile tested only, and definitely needs to be chunked into multiple patches,
> > > > > but I think something like this mess?
> > > > 
> > > > The proposed patch does not fix the problem. In fact, it messes things
> > > > up so much that I don't get any test results back.
> > > 
> > > Drat.
> > > 
> > > > Google has an internal K-U-T test that demonstrates the problem. I
> > > > will post it soon.
> > > 
> > > Received, I'll dig in soonish, though "soonish" might unfortunately might mean
> > > 2024.
> > > 
> > 
> > Hi,
> > 
> > So this is what I think:
> > 
> > 
> > KVM does have kvm_guest_apic_has_interrupt() for this exact purpose,
> > to check if nested APICv has a pending interrupt before halting.
> > 
> > 
> > However the problem is bigger - with APICv we have in essence 2 pending interrupt
> > bitmaps - the PIR and the IRR, and to know if the guest has a pending interrupt
> > one has in theory to copy PIR to IRR, then see if the max is larger then the current PPR.
> > 
> > Since we don't want to write to guest memory, and the IRR here resides in the guest memory,
> > I guess we have to do a 'dry-run' version of 'vmx_complete_nested_posted_interrupt' and call
> > it from  kvm_guest_apic_has_interrupt().
> > 
> > What do you think? I can prepare a patch for this.
> > 
> > Can you share a reproducer or write a new one that can be shared?
> 
> See https://lore.kernel.org/kvm/20231211185552.3856862-1-jmattson@google.com/.

Thank you!

Best regards,
	Maxim Levitsky

> 
> > Best regards,
> >         Maxim Levitsky
> >