Re: [PATCH v2 02/11] KVM: nSVM: clean up the copying of V_INTR bits from vmcb02 to vmcb12

[Sean Christopherson]

On Sat, Jan 28, 2023, Sean Christopherson wrote:
> On Tue, Nov 29, 2022, Maxim Levitsky wrote:
> > the V_IRQ and v_TPR bits don't exist when virtual interrupt
> > masking is not enabled, therefore the KVM should not copy these
> > bits regardless of V_IRQ intercept.
> 
> Hmm, the APM disagrees:
> 
>  The APIC's TPR always controls the task priority for physical interrupts, and the
>  V_TPR always controls virtual interrupts.
> 
>    While running a guest with V_INTR_MASKING cleared to 0:
>      • Writes to CR8 affect both the APIC's TPR and the V_TPR register.
> 
> 
>  ...
> 
>  The three VMCB fields V_IRQ, V_INTR_PRIO, and V_INTR_VECTOR indicate whether there
>  is a virtual interrupt pending, and, if so, what its vector number and priority are.
> 
> IIUC, V_INTR_MASKING_MASK is mostly about EFLAGS.IF, with a small side effect on
> TPR.  E.g. a VMM could pend a V_IRQ but clear V_INTR_MASKING and expect the guest
> to take the V_IRQ.  At least, that's my reading of things.
>
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/svm/nested.c | 23 ++++++++---------------
> >  1 file changed, 8 insertions(+), 15 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index 37af0338da7c32..aad3145b2f62fe 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -412,24 +412,17 @@ void nested_copy_vmcb_save_to_cache(struct vcpu_svm *svm,
> >   */
> >  void nested_sync_control_from_vmcb02(struct vcpu_svm *svm)
> >  {
> > -	u32 mask;
> > +	u32 mask = 0;
> >  	svm->nested.ctl.event_inj      = svm->vmcb->control.event_inj;
> >  	svm->nested.ctl.event_inj_err  = svm->vmcb->control.event_inj_err;
> >  
> > -	/* Only a few fields of int_ctl are written by the processor.  */
> > -	mask = V_IRQ_MASK | V_TPR_MASK;
> > -	if (!(svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK) &&
> > -	    svm_is_intercept(svm, INTERCEPT_VINTR)) {
> > -		/*
> > -		 * In order to request an interrupt window, L0 is usurping
> > -		 * svm->vmcb->control.int_ctl and possibly setting V_IRQ
> > -		 * even if it was clear in L1's VMCB.  Restoring it would be
> > -		 * wrong.  However, in this case V_IRQ will remain true until
> > -		 * interrupt_window_interception calls svm_clear_vintr and
> > -		 * restores int_ctl.  We can just leave it aside.
> > -		 */
> > -		mask &= ~V_IRQ_MASK;

Argh! *shakes fist at KVM and SVM*

This is ridiculously convoluted, and I'm pretty sure there are existing bugs.  If
L1 runs L2 with V_IRQ=1 and V_INTR_MASKING=1, and KVM requests an interrupt window,
then KVM will overwrite vmcb02's int_vector and int_ctl, i.e. clobber L1's V_IRQ,
but then silently clear INTERCEPT_VINTR in recalc_intercepts() and thus prevent
svm_clear_vintr() from being reached, i.e. prevent restoring L1's V_IRQ.

Bug #1 is that KVM shouldn't clobber the V_IRQ fields if KVM ultimately decides
not to open an interrupt window.  Bug #2 is that KVM needs to open an interrupt
window if save.RFLAGS.IF=1, as interrupts may become unblocked in that case,
e.g. if L2 is in an interrupt shadow.

So I think this over two patches?

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 05d38944a6c0..ad1e70ac8669 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -139,13 +139,18 @@ void recalc_intercepts(struct vcpu_svm *svm)
 
        if (g->int_ctl & V_INTR_MASKING_MASK) {
                /*
-                * Once running L2 with HF_VINTR_MASK, EFLAGS.IF and CR8
-                * does not affect any interrupt we may want to inject;
-                * therefore, writes to CR8 are irrelevant to L0, as are
-                * interrupt window vmexits.
+                * If L2 is active and V_INTR_MASKING is enabled in vmcb12,
+                * disable intercept of CR8 writes as L2's CR8 does not affect
+                * any interrupt KVM may want to inject.
+                *
+                * Similarly, disable intercept of virtual interrupts (used to
+                * detect interrupt windows) if the saved RFLAGS.IF is '0', as
+                * the effective RFLAGS.IF for L1 interrupts will never be set
+                * while L2 is running (L2's RFLAGS.IF doesn't affect L1 IRQs).
                 */
                vmcb_clr_intercept(c, INTERCEPT_CR8_WRITE);
-               vmcb_clr_intercept(c, INTERCEPT_VINTR);
+               if (!(svm->vmcb01.ptr->save.rflags & X86_EFLAGS_IF))
+                       vmcb_clr_intercept(c, INTERCEPT_VINTR);
        }
 
        /*
@@ -416,18 +421,18 @@ void nested_sync_control_from_vmcb02(struct vcpu_svm *svm)
 
        /* Only a few fields of int_ctl are written by the processor.  */
        mask = V_IRQ_MASK | V_TPR_MASK;
-       if (!(svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK) &&
-           svm_is_intercept(svm, INTERCEPT_VINTR)) {
-               /*
-                * In order to request an interrupt window, L0 is usurping
-                * svm->vmcb->control.int_ctl and possibly setting V_IRQ
-                * even if it was clear in L1's VMCB.  Restoring it would be
-                * wrong.  However, in this case V_IRQ will remain true until
-                * interrupt_window_interception calls svm_clear_vintr and
-                * restores int_ctl.  We can just leave it aside.
-                */
+
+       /*
+        * Don't sync vmcb02 V_IRQ back to vmcb12 if KVM (L0) is intercepting
+        * virtual interrupts in order to request an interrupt window, as KVM
+        * has usurped vmcb02's int_ctl.  If an interrupt window opens before
+        * the next VM-Exit, svm_clear_vintr() will restore vmcb12's int_ctl.
+        * If no window opens, V_IRQ will be correctly preserved in vmcb12's
+        * int_ctl (because it was never recognized while L2 was running).
+        */
+       if (svm_is_intercept(svm, INTERCEPT_VINTR) &&
+           !test_bit(INTERCEPT_VINTR, (unsigned long *)svm->nested.ctl.intercepts))
                mask &= ~V_IRQ_MASK;
-       }
 
        if (nested_vgif_enabled(svm))
                mask |= V_GIF_MASK;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index b103fe7cbc82..59d2891662ef 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1580,6 +1580,16 @@ static void svm_set_vintr(struct vcpu_svm *svm)
 
        svm_set_intercept(svm, INTERCEPT_VINTR);
 
+       /*
+        * Recalculating intercepts may have clear the VINTR intercept.  If
+        * V_INTR_MASKING is enabled in vmcb12, then the effective RFLAGS.IF
+        * for L1 physical interrupts is L1's RFLAGS.IF at the time of VMRUN.
+        * Requesting an interrupt window if save.RFLAGS.IF=0 is pointless as
+        * interrupts will never be unblocked while L2 is running.
+        */
+       if (!svm_is_intercept(svm, INTERCEPT_VINTR))
+               return;
+
        /*
         * This is just a dummy VINTR to actually cause a vmexit to happen.
         * Actual injection of virtual interrupts happens through EVENTINJ.