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Re: [Xen-devel] [PATCH for-4.5] x86/HVM: Partial revert of 28b4baacd5

>>> On 25.11.14 at 11:46, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 25/11/14 10:42, Jan Beulich wrote:
>>>>> On 25.11.14 at 11:08, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> A failed vmentry is overwhelmingly likely to be caused by corrupt VMCS 
> state.
>>> As a result, injecting a fault and retrying the the vmentry is likely to 
>>> fail
>>> in the same way.
>> That's not all that unlikely - remember that the change was prompted
>> by the XSA-110 fix. There CS pieces being in a bad state would get
>> corrected by the exception injection.
>>> One other alternative, which I would pursue if we were not already in -rc2
>>> would be to add some extra logic to detect repeated vmentry failure and 
>>> allow
>>> one attempt to shoot userspace before giving up and crashing the domain.
>> That's not even needed afaict (and if it really is, it can't be all that
>> difficult/intrusive): Did you observe what you attempt to fix here in
>> practice, or is this just from theoretical considerations? I ask because
>> I don't think it can actually happen, as the second time we get here
>> the guest ought to be in kernel mode (due to the exception injection)
>> and hence would get crashed anyway.
> Only from theoretical considerations.  A bad CS (and possibly SS) would
> be fixed by this, but there are many others which wouldn't

But that doesn't eliminate the fact that in the second pass we'd find
the guest in kernel mode, and hence crash it. Yet your reply sounds
as if you still think your patch is needed.


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