Re: [PATCHv1 3/5] x86/fpu: Add a per-domain field to set the width of FIP/FDP

[David Vrabel <david.vrabel@citrix.com>]

On 18/02/16 19:13, Andrew Cooper wrote:
> On 18/02/16 18:52, David Vrabel wrote:
>> --- a/xen/include/asm-x86/domain.h
>> +++ b/xen/include/asm-x86/domain.h
>> @@ -394,6 +394,21 @@ struct arch_domain
>>  
>>      /* Emulated devices enabled bitmap. */
>>      uint32_t emulation_flags;
>> +
>> +    /*
>> +     * The width of the FIP/FDP register in the FPU that needs to be
>> +     * saved/restored during a context switch.  This is needed because
>> +     * the FPU can either: a) restore the 64-bit FIP/FDP and clear FCS
>> +     * and FDS; or b) retstore the 32-bit FIP/FDP (clearing the upper
>> +     * 32-bits) and restore FCS/FDS.
>> +     *
>> +     * Which one is needed depends on the guest.
>> +     *
>> +     * This can be either: 8, 4 or 0.  0 means auto-detect the size
>> +     * based on the width of FIP/FDP values that are written by by the
>> +     * guest.
>> +     */
>> +    uint8_t x87_fip_width;
> 
> Can we get away with always using fpu_sse.x[FPU_WORD_SIZE_OFFSET],
> instead if duplicating the information in arch_domain and risking them
> getting out of sync?

No.  Because if x87_fip_width == 0 (auto-mode) we check every save for
the correct width to use, storing the result at FPU_WORD_SIZE_OFFSET.

> VMs which have migrated in will already have some policy latched, and we
> should preserve their old behaviour if possible.

This does.  x87_fip_width is only used on save. The restore behaviour is
unchanged and is conditional on the word size written to the save state.

David