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