Re: [Xen-devel] Emulation and active (valid) interrupts

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.08.18 at 15:19, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 8/13/18 3:58 PM, Jan Beulich wrote:
>>>>> On 13.08.18 at 14:51, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> So first we've got that vmx_idtv_reinject() call writing to the VMCS,
>>> then we emulate a CLI, then the failed vmentry. I can't tell if the CLI
>>> ran first and then an interrupt popped up, or if an interrupt had
>>> already been __vmwrit()ten and then CLI caused the invalid guest state.
>> 
>> I'd expect it to be the latter - an external interrupt presumably
>> can't be injected when EFLAGS.IF is clear. Why are we emulating
>> CLI in the first place? With a pending external interrupt, shouldn't
>> we just exit back to guest context without emulating anything?
> 
> In this particular case we're emulating CLI because the vm_event
> response requests it.
> 
> Tamas' test marks all of the guest's pages XENMEM_access_x, and at some
> point a vm_event arrives somewhere in a page where CLI is read from,
> AFAICT. Doing nothing would get us into an infinite loop, and since we
> don't want to mark the page rwx, we try to emulate CLI.

Doing nothing would get you into an infinite loop only if at each
attempt there's yet again an event to be re-injected. Of course
the risk of this grows the longer it takes to processes things in
your tool, but if there is an event to be re-injected then I don't
see what else you can do. Trying to ditch the event would
certainly be the wrong thing. I suggest you try to get advice
from the VMX maintainers - perhaps I'm simply overlooking an
obvious route out of the state you're apparently in.

Jan



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