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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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()

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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