Re: [PATCH v7 04/27] x86/fpu/xstate: Introduce XSAVES system states

Dave Hansen
2019-06-06T22:08:25Z

On 6/6/19 3:04 PM, Andy Lutomirski wrote:
>> But, that seems broken. If we have supervisor state, we can't
>> always defer the load until return to userspace, so we'll never??
>> have TIF_NEED_FPU_LOAD. That would certainly be true for
>> cet_kernel_state.
>
> Ugh. I was sort of imagining that we would treat supervisor state
completely separately from user state. But can you maybe give
examples of exactly what you mean?
>
>> It seems like we actually need three classes of XSAVE states: 1.
>> User state
>
> This is FPU, XMM, etc, right?

Yep.

>> 2. Supervisor state that affects user mode
>
> User CET?

Yep.

>> 3. Supervisor state that affects kernel mode
>
> Like supervisor CET? If we start doing supervisor shadow stack, the
> context switches will be real fun. We may need to handle this in
> asm.

Yeah, that's what I was thinking.

I have the feeling Yu-cheng's patches don't comprehend this since
Sebastian's patches went in after he started working on shadow stacks.

> Where does PKRU fit in? Maybe we can treat it as #3?

I thought Sebastian added specific PKRU handling to make it always
eager. It's actually user state that affect kernel mode. :)