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

[Sean Christopherson <seanjc@google.com>]

On Fri, May 26, 2023, Alexey Kardashevskiy wrote:
> 
> On 24/5/23 01:44, Sean Christopherson wrote:
> > On Tue, May 23, 2023, Alexey Kardashevskiy wrote:
> > > > Actually, can't disabling #DB interception for DebugSwap SEV-ES guests be a
> > > > separate patch?  KVM can still inject #DBs for SEV-ES guests, no?
> > > 
> > > Sorry for my ignorance but what is the point of injecting #DB if there is no
> > > way of changing the guest's DR7?
> > 
> > Well, _injecting_ the #DB is necessary for correctness from the guest's perspective.
> > "What's the point of _intercepting_ #DB" is the real question.  And for SEV-ES guests
> > with DebugSwap, there is no point, which is why I agree that KVM should disable
> > interception in that case.  What I'm calling out is that disabling #Db interception
> > isn't _necessary_ for correctness (unless I'm missing something), which means that
> > it can and should go in a separate patch.
> 
> 
> About this. Instead of sev_es_init_vmcb(), I can toggle the #DB intercept
> when toggling guest_debug, see below. This
> kvm_x86_ops::update_exception_bitmap hook is called on vcpu reset and
> kvm_arch_vcpu_ioctl_set_guest_debug (which skips this call if
> guest_state_protected = true).

KVM also intercepts #DB when single-stepping over IRET to find an NMI window, so
you'd also have to factor in nmi_singlestep, and update svm_enable_nmi_window()
and disable_nmi_singlestep() to call svm_update_exception_bitmap().

> Is there any downside?

Complexity is the main one.  The complexity is quite low, but the benefit to the
guest is likely even lower.  A #DB in the guest isn't likely to be performance
sensitive.  And on the flip side, opening an NMI window would be a tiny bit more
expensive, though I doubt that would be meaningful either.

All in all, I think it makes sense to just keep intercepting #DB for non-SEV-ES
guests.

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.  Blech,
and suppressing EFER.SVME in efer_trap() is a bit gross, but I suppose since the
GHCB doesn't allow for CLGI or STGI it's "fine".

E.g. shouldn't KVM do this?

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)
        if (svm_get_nmi_mask(vcpu) && !svm->awaiting_iret_completion)
                return; /* IRET will cause a vm exit */
 
+       /*
+        * KVM can't single-step SEV-ES guests and instead assumes that IRET
+        * in the guest will always succeed, i.e. clears NMI masking on the
+        * next VM-Exit.  Note, GIF is guaranteed to be '1' for SEV-ES guests
+        * as the GHCB doesn't allow for CLGI or STGI (and KVM suppresses
+        * EFER.SVME for good measure, see efer_trap()).
+        */
+       if (sev_es_guest(vcpu->kvm))
+               return;
+
        if (!gif_set(svm)) {
                if (vgif)
                        svm_set_intercept(svm, INTERCEPT_STGI);