> Hmm, if the guest runs an infinite emulated 'mov ss', it will keep
> toggling the MOV_SS bit, but STI will remain set, so we'll never allow
> an interrupt into the guest kernel.
We have no choice but returning both flags, since svm does not differentiate
between them.
But see below for an alternative path that makes it a non-issue.
>
>> diff --git a/arch/x86/kvm/x86_emulate.c b/arch/x86/kvm/x86_emulate.c
>> index d2664fc..797d41f 100644
>> --- a/arch/x86/kvm/x86_emulate.c
>> +++ b/arch/x86/kvm/x86_emulate.c
>> @@ -1618,6 +1618,16 @@ special_insn:
>> int err;
>> sel = c->src.val;
>> + if (c->modrm_reg == VCPU_SREG_SS) {
>> + u32 int_shadow =
>> + kvm_x86_ops->get_interrupt_shadow(ctxt->vcpu);
>> + /* See sti emulation for an explanation of this */
>> + if ((int_shadow & X86_SHADOW_INT_MOV_SS))
>> + ctxt->interruptibility &= ~X86_SHADOW_INT_MOV_SS;
>> + else
>> + ctxt->interruptibility |= X86_SHADOW_INT_MOV_SS;
>> + }
>>
>
> ^=
=p \o/
After re-reading this, masking the flags in here makes no sense.
I am moving to an approach in which I do
if (!(int_shadow & X86_SHADOW_INT_MOV_SS))
ctxt->interruptibility = X86_SHADOW_INT_MOV_SS;
Since if the next instruction is an sti, it is certainly not an sti; sti instruction
(the current is mov ss, after all). So we should mask it anyway. This also solves
nicely the problem you raised at svm.c.
>
>> @@ -1846,10 +1856,23 @@ special_insn:
>> ctxt->eflags &= ~X86_EFLAGS_IF;
>> c->dst.type = OP_NONE; /* Disable writeback. */
>> break;
>> - case 0xfb: /* sti */
>> + case 0xfb: { /* sti */
>> + u32 int_shadow = kvm_x86_ops->get_interrupt_shadow(ctxt->vcpu);
>> + /*
>> + * an sti; sti; sequence only disable interrupts for the first
>> + * instruction. So, if the last instruction, be it emulated or
>> + * not, left the system with the INT_STI flag enabled, it
>> + * means that the last instruction is an sti. We should not
>> + * leave the flag on in this case
>> + */
>> + if ((int_shadow & X86_SHADOW_INT_STI))
>> + ctxt->interruptibility &= ~X86_SHADOW_INT_STI;
>> + else
>> + ctxt->interruptibility |= X86_SHADOW_INT_STI;
>>
>
> ^=
ditto
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