Re: [Xen-devel] [PATCHv2 1/3] x86/fpu: improve check for XSAVE* not writing FIP/FDP fields

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 23/02/16 11:18, Andrew Cooper wrote:
> On 23/02/16 11:05, David Vrabel wrote:
>> The hardware may not write the FIP/FDP fields with a XSAVE*
>> instruction.  e.g., with XSAVEOPT/XSAVES if the state hasn't changed
>> or on AMD CPUs when a floating point exception is not pending.  We
>> need to identify this case so we can correctly apply the check for
>> whether to save/restore FCS/FDS.
>>
>> By toggling FIP[63] we can turn the field into a non-canonical address
>> and check for this value after the XSAVE instruction.
>>
>> This results in smaller code with fewer branches and is more
>> understandable.
>>
>> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> For consistently, the same change in detection logic should be applied
> to fpu_fxsave()

I don't think it is necessary.  fpu_fxsave() only needs to check for the
AMD case, and the logic is already simple.

David

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