Denys Vlasenko <dvlas...@redhat.com> writes:
> On 03/23/2015 04:30 AM, Rusty Russell wrote:
>> + * They may be about to iret, where they asked us never to
>> + * deliver interrupts. In this case, we can emulate that iret
>> + * then immediately deliver the interrupt. This is bascially
>> + * a noop: the iret would pop the interrupt frame and restore
>> + * eflags, and then we'd set it up again. So just restore the
>> + * eflags word and jump straight to the handler in this case.
>> */
>> + if (cpu->regs->eip >= cpu->lg->noirq_start &&
>> + (cpu->regs->eip < cpu->lg->noirq_end)) {
>> + restore_eflags(cpu);
>
> In truth, this is not _exactly_ true for irets to CPL3.
>
> If a new interrupt comes right after iret, then
> a new transition to CPL0 will happen.
>
> This means ss:esp will be loaded from tss.ss0:tss.sp0.
>
> Meaning, that the new iret frame may be in a different place
> than the one which was used by iret.
True. We could check the to-be-restored-CPL and reset the sp. Instead,
I've added this comment:
/*
* They may be about to iret, where they asked us never to
* deliver interrupts. In this case, we can emulate that iret
* then immediately deliver the interrupt. This is basically
* a noop: the iret would pop the interrupt frame and restore
* eflags, and then we'd set it up again. So just restore the
* eflags word and jump straight to the handler in this case.
*
* Denys Vlasenko points out that this isn't quite right: if
* the iret was returning to userspace, then that interrupt
* would reset the stack pointer (which the Guest told us
* about via LHCALL_SET_STACK). But unless the Guest is being
* *really* weird, that will be the same as the current stack
* anyway.
*/
> There is no good reason for CPL0 code to move iret frame around,
> but who knows. As an example, look what 32-bit Linux kernel does
> with NMI iret frames... it's mind bending.
Fortunately, lguest is allergic to NMIs :)
Thanks!
Rusty.
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