Re: [Xen-devel] [PATCHv3 1/3] x86/fpu: improve check for XSAVE* not writing FIP/FDP fields

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 25/02/16 15:07, Andrew Cooper wrote:
> On 25/02/16 14:27, Jan Beulich wrote:
>>>>> On 25.02.16 at 14:16, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> No OS is plausibly going to hide non-IP information in FIP.
>>>
>>> If some theoretical OS does do something like that, there is always the
>>> override available.
>> But who says it needs to be an OS to do so? User mode code could
>> as much (and would imo even be more likely to do so, compared to
>> an OS).
> 
> Why does this matter?  We *cannot* make a perfect heuristic in this case.
> 
> The new heuristic is wrong substantially less often than the old
> heuristic, so is an improvement.

Well, no.  The current one is wrong 0.01% of the time[1], and is now
wrong 0.0100000000000000001% of the time.  I would suggest that this
isn't a significant different.

Anyway, since it was trivial to reorder the series, I'm posting a v4
with this controversial patch at the end.

David

[1] Based on the number of XenServer test cases it breaks.


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