Re: [PATCH] disable interrupt shadow state for emulated instruction

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Glauber Costa wrote:
we currently unblock shadow interrupt state when we skip an instruction,
but failing to do so when we actually emulate one. This blocks interrupts
in key instruction blocks, in particular sti; hlt; sequences

Without this patch, I cannot boot gpxe option roms at vmx machines.
This is described at https://bugzilla.redhat.com/show_bug.cgi?id=494469

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index c6997c0..cee38e4 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -736,26 +736,34 @@ static void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
 	vmcs_writel(GUEST_RFLAGS, rflags);
 }
+static void vmx_block_interrupt_shadow(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * We emulated an instruction, so temporary interrupt blocking
+	 * should be removed, if set.
+	 */
+	u32 interruptibility = vmcs_read32(GUEST_INTERRUPTIBILITY_INFO);
+	u32 interruptibility_mask = ((GUEST_INTR_STATE_STI | GUEST_INTR_STATE_MOV_SS));
+
+	if (interruptibility & interruptibility_mask)
+		vmcs_write32(GUEST_INTERRUPTIBILITY_INFO,
+			     interruptibility & ~interruptibility_mask);
+	vcpu->arch.interrupt_window_open = 1;
+}
+

How does this logic work when the instruction emulated is an STI or MOV SS instruction? In particular, when does GUEST_INTERRUPTIBILITY_INFO sets set to reflect the *blocking* operation?

The pseudo-code for this kind of stuff looks like:


forever {
	tmp_int_flags <- int_flags

	/* Begin instruction execution */
	int_flags |= GUEST_INTR_STATE_STI	/* STI instruction */
	/* End instruction execution */

	int_flags &= ~tmp_int_flags

	if (irq_pending && eflags.if == 1 && int_flags == 0)
		take_interrupt();
}

Note the behavior in the case of sequential STIs, that int_flags goes to 0 after the second execution.

	-hpa
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