Re: [Xen-devel] [PATCH v2] x86/PV: don't wrongly hide/expose CPUID.OSXSAVE from/to user mode

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.08.16 at 11:24, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 23/08/16 10:00, Jan Beulich wrote:
>>>>> On 22.08.16 at 19:30, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 19/08/16 19:07, Andrew Cooper wrote:
>>>> On 19/08/16 18:09, Andrew Cooper wrote:
>>>>> On 19/08/16 13:53, Jan Beulich wrote:
>>>>>> User mode code generally cannot be expected to invoke the PV-enabled
>>>>>> CPUID Xen supports, and prior to the CPUID levelling changes for 4.7
>>>>>> (as well as even nowadays on levelling incapable hardware) such CPUID
>>>>>> invocations actually saw the host CR4.OSXSAVE value, whereas prior to
>>>>>> this patch
>>>>>> - on Intel guest user mode always saw the flag clear,
>>>>>> - on AMD guest user mode saw the flag set even when the guest kernel
>>>>>>   didn't enable use of XSAVE/XRSTOR.
>>>>>> Fold in the guest view of CR4.OSXSAVE when setting the levelling MSRs,
>>>>>> just like we do in other CPUID handling.
>>>>>>
>>>>>> To make guest CR4 changes immediately visible via CPUID, also invoke
>>>>>> ctxt_switch_levelling() from the CR4 write path.
>>> I was just putting together a patch series to make these changes in a
>>> more consistent manor, and have found a spanner in the works.
>>>
>>> Hiding Xen's view of OSXSAVE from a guests native cpuid will break
>>> Linux, because of the pile of hacks making up the current PV XSAVE support.
>> No, because PV Linux doesn't use native CPUID. We clearly should
>> not hide OSXSAVE in the PV variant (and your earlier series did take
>> great care to avoid that).
> 
> Where? There is no distinction for OSXSAVE.
> 
> The only place in pv_cpuid() which distinguishes native vs emulated
> cpuid is the X86_FEATURE_MONITOR handling.
> 
> We distinguish kernel and userspace quite a lot, so the compatibility
> breakages are only visible to the kernel.

That's actually the part I meant.

Jan


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