linux-kernel - Re: [PATCH kernel v5 5/6] KVM: SEV: Enable data breakpoints in SEV-ES

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Date:   Wed, 14 Jun 2023 14:27:49 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Alexey Kardashevskiy <aik@....com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Pankaj Gupta <pankaj.gupta@....com>,
        Nikunj A Dadhania <nikunj@....com>,
        Santosh Shukla <santosh.shukla@....com>,
        Carlos Bilbao <carlos.bilbao@....com>
Subject: Re: [PATCH kernel v5 5/6] KVM: SEV: Enable data breakpoints in SEV-ES

On Wed, Jun 14, 2023, Alexey Kardashevskiy wrote:
> On 14/6/23 09:19, Sean Christopherson wrote:
> > On Fri, Jun 02, 2023, Alexey Kardashevskiy wrote:
> > > > > Side topic, isn't there an existing bug regarding SEV-ES NMI windows?
> > > > > KVM can't actually single-step an SEV-ES guest, but tries to set
> > > > > RFLAGS.TF anyways.
> > > > =20
> > > > Why is it a "bug" and what does the patch fix? Sound to me as it is
> > > > pointless and the guest won't do single stepping and instead will run
> > > > till it exits somehow, what do I miss?
> > 
> > The bug is benign in the end, but it's still a bug.  I'm not worried about =
> 
> 
> (unrelated) Your response's encoding broke somehow and I wonder if this is
> something I did or you did. Lore got it too:
> 
> https://lore.kernel.org/all/ZIj5ms+DohcLyXHE@google.com/

Huh.  Guessing something I did, but I've no idea what caused it.

> > fixing
> > any behavior, but I dislike having dead, misleading code, especially for so=
> > mething
> > like this where both NMI virtualization and SEV-ES are already crazy comple=
> > x and
> > subtle.  I think it's safe to say that I've spent more time digging through=
> >   SEV-ES
> > and NMI virtualization than most KVM developers, and as evidenced by the nu=
> > mber of
> > things I got wrong below, I'm still struggling to keep track of the bigger =
> > picture.
> > Developers that are new to all of this need as much help as they can get.
> > 
> > > > > Blech, and suppressing EFER.SVME in efer_trap() is a bit gross,
> > > > =20
> > > > Why suppressed? svm_set_efer() sets it eventually anyway.
> > 
> > svm_set_efer() sets SVME in hardware, but KVM's view of the guest's value t=
> > hat's
> > stored in vcpu->arch.efer doesn't have SVME set.  E.g. from the guest's per=
> > spective,
> > EFER.SVME will have "Reserved Read As Zero" semantics.
> 
> It is not zero, why? From inside the guest, rdmsrl(MSR_EFER, efer) reads
> 0x1d01 from that msr where 0x1000==(1<<_EFER_SVME),  _EFER_SVME==12.

Oh, lame.  So the guest gets to see the raw value in the VMSA.  So it really comes
down to the GHCB not providing support for STGI/CLGI.