Re: [patch 01/10] x86/fpu/signal: Clarify exception handling in restore_fpregs_from_user()

[Thomas Gleixner]

Sean,

On Wed, Sep 01 2021 at 16:47, Sean Christopherson wrote:
> On Wed, Sep 01, 2021, Thomas Gleixner wrote:
>> Though while for the FPU use case we really want to handle the #MC case,
>> it's not clear to me whether this is actually correct for SGX.
>> 
>> Jarkko, Sean, Dave?
>
> Are you asking about #MC specifically, or about SGX consuming the trap number in
> general?

#MC

> For #MC, it's probably a moot point because #MC on ENCLS is not recoverable for
> current hardware.  If #MC somehow occurs on ENCLS and doesn't kill the platform,
> "handling" the #MC in SGX is probably wrong.  Note, Tony is working on a series to
> support recoverable #MC on SGX stuff on future hardware[*], but I'm not sure that's
> relevant to this discussion.

Probably not.

> As for SGX consuming the trap number in general, it's correct.  For non-KVM usage,
> it's nice to have but not strictly necessary.  Any fault except #PF on ENCLS is
> guaranteed to be a kernel or hardware bug; SGX uses the trap number to WARN on a
> !#PF exception, e.g. on #GP or #UD.  Not having the trap number would mean losing
> those sanity checks, which have been useful in the past.
>
> For virtualizing SGX, KVM needs the trap number so that it can inject the correct
> exception into the guest, e.g. if the guest violates the ENCLS concurrency rules
> it should get a #GP, whereas a EPCM violation should #PF.

Yes, I understood that part, but I was confused about the #MC part.

Thanks for clarifying!

       tglx