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

["H. Peter Anvin" <hpa@zytor.com>]

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