On 29/04/2024 8:49 am, Jan Beulich wrote:
> On 26.04.2024 18:08, Andrew Cooper wrote:
>> Some features need accumulating rather than intersecting to make migration
>> safe. Introduce the new '|' attribute for this purpose.
>>
>> Right now, it's only used by the Xapi toolstack, but it will be used by
>> xl/libxl when the full policy-object work is complete, and until then it's
>> still a useful hint for hand-crafted cpuid= lines in vm.cfg files.
>>
>> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
> Reviewed-by: Jan Beulich <jbeul...@suse.com>
Thanks.
> The one feature you don't annotate such that I'm not entirely sure about is
> NO_FPU_SEL: On one hand it tells code not to rely on / use the selector
> fields in FPU state.
It's sadly far more complicated than this.
This feature, and it's AMD partner RSTR_FP_ERR_PTRS, where introduced to
stop windows BSOD-ing under virt, and came with an accidental breakage
of x86emul/DoSBox/etc which Intel and AMD have declined to fix.
If you recall, prior to these features, the hypervisor needs to divine
the operand size of Window's {F,}X{SAVE,RESTORE} instructions, as it
blindly does a memcmp() across the region to confirm that the interrupt
handler didn't corrupt any state.
Both features are discontinuities across which is is not safe to migrate.
Advertising "reliably store as 0" on older systems will still cause
windows to BSOD on occasion, whereas not advertising it on newer systems
will suggest that legacy emulators will work, when they don't.
I don't have any good idea of how to make this work nicely, other than
having some admin booleans in the xl.cfg file saying "I certify I'm not
using a legacy emulator in my VM" to override the safety check.
Other discontinuities I'm aware of are the introduction of DAZ (changing
MXCSR_MASK), and any migration which changes LBR_FORMAT.
~Andrew
