Re: [PATCH v5 09/19] KVM:x86: Make guest supervisor states as non-XSAVE managed

["Yang, Weijiang" <weijiang.yang@intel.com>]

On 8/29/2023 5:00 AM, Dave Hansen wrote:
> On 8/10/23 08:15, Paolo Bonzini wrote:
>> On 8/10/23 16:29, Dave Hansen wrote:
>>> What actual OSes need this support?
>> I think Xen could use it when running nested.  But KVM cannot expose
>> support for CET in CPUID, and at the same time fake support for
>> MSR_IA32_PL{0,1,2}_SSP (e.g. inject a #GP if it's ever written to a
>> nonzero value).
>>
>> I suppose we could invent our own paravirtualized CPUID bit for
>> "supervisor IBT works but supervisor SHSTK doesn't".  Linux could check
>> that but I don't think it's a good idea.
>>
>> So... do, or do not.  There is no try. :)
> Ahh, that makes sense.  This is needed for implementing the
> *architecture*, not because some OS actually wants to _do_ it.
>
> ...
>>> In a perfect world, we'd just allocate space for CET_S in the KVM
>>> fpstates.  The core kernel fpstates would have
>>> XSTATE_BV[13]==XCOMP_BV[13]==0.  An XRSTOR of the core kernel fpstates
>>> would just set CET_S to its init state.
>> Yep.  I don't think it's a lot of work to implement.  The basic idea as
>> you point out below is something like
>>
>> #define XFEATURE_MASK_USER_DYNAMIC XFEATURE_MASK_XTILE_DATA
>> #define XFEATURE_MASK_USER_OPTIONAL \
>>      (XFEATURE_MASK_DYNAMIC | XFEATURE_MASK_CET_KERNEL)
>>
>> where XFEATURE_MASK_USER_DYNAMIC is used for xfd-related tasks
>> (including the ARCH_GET_XCOMP_SUPP arch_prctl) but everything else uses
>> XFEATURE_MASK_USER_OPTIONAL.
>>
>> KVM would enable the feature by hand when allocating the guest fpstate.
>> Disabled features would be cleared from EDX:EAX when calling
>> XSAVE/XSAVEC/XSAVES.
> OK, so let's _try_ this perfect-world solution.  KVM fpstates get
> fpstate->xfeatures[13] set, but no normal task fpstates have that bit
> set.  Most of the infrastructure should be there to handle this without
> much fuss because it _should_ be looking at generic things like
> fpstate->size and fpstate->features.
>
> But who knows what trouble this will turn up.  It could get nasty and
> not worth it, but we should at least try it.

Thanks Dave for clarity!
I'm moving in that direction...