Message-ID: <ZIj5ms+DohcLyXHE@google.com>
Date:   Tue, 13 Jun 2023 16:19:54 -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 Fri, Jun 02, 2023, Alexey Kardashevskiy wrote:
> Sean, ping?
>=20
> I wonder if this sev-es-not-singlestepping is a showstopper or it is alri=
ght
> to repost this patchset without it? Thanks,

Ah, shoot, I completely lost this in my inbox.  Sorry :-/

> > > 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 =
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.

> > > but I suppose since the GHCB doesn't allow for CLGI or STGI it's "fin=
e".
> >=20
> > GHCB does not mention this, instead these are always intercepted in
> > init_vmcb().

Right, I'm calling out that the absense of protocol support for requesting =
CLGI
or STGI emulation means dropping the guest's EFER.SVME is ok (though gross =
:-) ).

> > > E.g. shouldn't KVM do this?
> >=20
> > It sure can and I am happy to include this into the series, the commit
> > log is what I am struggling with :)
> >=20
> > >=20
> > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > > index ca32389f3c36..4e4a49031efe 100644
> > > --- a/arch/x86/kvm/svm/svm.c
> > > +++ b/arch/x86/kvm/svm/svm.c
> > > @@ -3784,6 +3784,16 @@ static void svm_enable_nmi_window(struct
> > > kvm_vcpu *vcpu)
> > > =EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=
=BD if (svm_get_nmi_mask(vcpu) && !svm->awaiting_iret_completion)
> > > =EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=
=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=
 return; /* IRET will cause a vm exit */
> > > +=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD /*
> > > +=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD * KV=
M can't single-step SEV-ES guests and instead assumes
> > > that IRET
> > > +=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD * in=
 the guest will always succeed,
> >=20
> > It relies on GHCB's NMI_COMPLETE (which SVM than handles is it was IRET=
):
> >=20
> >  =EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD case S=
VM_VMGEXIT_NMI_COMPLETE:
> >  =EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=
=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD ret =3D =
svm_invoke_exit_handler(vcpu, SVM_EXIT_IRET);
> >  =EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=
=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD break;

Ah, right, better to say that the guest is responsible for signaling that i=
t's
ready to accept NMIs, which KVM handles by "emulating" IRET.

> > > i.e. clears NMI masking on the
> > > +=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD * ne=
xt VM-Exit.=EF=BF=BD Note, GIF is guaranteed to be '1' for
> > > SEV-ES guests
> > > +=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD * as=
 the GHCB doesn't allow for CLGI or STGI (and KVM suppresses
> > > +=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD=EF=BF=BD * EF=
ER.SVME for good measure, see efer_trap()).
> >=20
> > SVM KVM seems to not enforce EFER.SVME, the guest does what it wants an=
d
> > KVM is only told the new value via EFER_WRITE_TRAP. And "writes by
> > SEV-ES guests to EFER.SVME are always ignored by hardware" says the APM=
.

Ahhh, that blurb in the APM is what I'm missing.

Actually, there's a real bug here.  KVM doesn't immediately unmask NMIs in =
response
to NMI_COMPLETE, and instead goes through the whole awaiting_iret_completio=
n =3D>
svm_complete_interrupts(), which means that KVM doesn't unmask NMIs until t=
he
*next* VM-Exit.  Theoretically, that could be never, e.g. if the host is ti=
ckless
and the guest is configured to busy wait idle CPUs.

Attached patches are compile tested only.

View attachment "3D"0001-KVM-SVM-Don-t-defer-NMI-unblocking-until-next-exit-f.patc=" of type "text/x-diff" (3011 bytes)

View attachment "3D"0002-KVM-SVM-Don-t-try-to-pointlessly-single-step-SEV-ES-.patc=" of type "text/x-diff" (1833 bytes)

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