On 14/6/23 09:19, Sean Christopherson wrote:
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 :-/
I saw the "OOO" message the other day and relaxed :)
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@xxxxxxxxxx/
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.
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.
Well, NMIs still get injected from QEMU so I guess it is a pass? Thanks,
--
Alexey
