Re: [Xen-devel] [PATCH] x86/HVM: extend LAPIC shortcuts around P2M lookups

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.08.14 at 21:53, <tim@xxxxxxx> wrote:
> At 08:12 +0100 on 04 Aug (1407136337), Jan Beulich wrote:
>> >>> On 01.08.14 at 21:15, <tim@xxxxxxx> wrote:
>> > If Xen does its own instruction fetch and decode, then we have to be
>> > careful about reusing any state from the original exit because of
>> > self-modifying code.  (And yes, that is a serious concern -- I once
>> > spent months trying to debug occasional memory corruption in the
>> > self-modifying license-enforcement code on a system stress test
>> > utility.)
>> > 
>> > So it would be OK to reuse the GPA from the exit if we could verify
>> > that the GVA we see is the same as the original fault (since there can't
>> > have been a TLB flush).  But IIRC the exit doesn't tell us the
>> > original GVA. :(
>> 
>> I don't think it needs to be as strict as this: For one, I wouldn't
>> intend to use the known GPA for instruction fetches at all. And
>> then I think if the instruction got modified between the exit and us
>> doing the emulation, using the known GPA with the wrong
>> instruction is as good or as bad as emulating an instruction that
>> didn't originally cause the exit.
> 
> Not at all -- as I said, in the shadow code we did see the case where
> we emulated a different instruction, and we do our best to handle it.
> And at least there we have a clean failure mode: if we can't emulate
> we crash.
> 
> Using the wrong GPA will silently corrupt memory and carry on, which
> is about the worst failure mode a VMM can have (esp. if skipping the
> GVA->GPA walk could allow a guest process to write to a read-only
> mapping).  

Indeed, thinking about it again I agree. Fortunately it looks like we're
having ways to accelerate this nevertheless: On EPT, the handler
gets the linear address, we just need to make use of it. I just finished
drafting a respective patch - hopefully I'll get to trying it out later
today.

> I'd be extremely uncomfortable with anything like tis unless there's a
> way to get either the ifetch buffer or a partial decode out of the CPU
> (which IIRC can't be done on x86 though it can on ARM).

On NPT we also get the instruction bytes on nested page faults, at
least on newer hardware. So maybe we could cook up something
along the lines you indicate by flagging that the instruction bytes
came from hardware.

Jan


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