Re: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

[Andy Lutomirski]

> On Oct 19, 2018, at 12:44 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> On 18/10/2018 22:46, Andy Lutomirski wrote:
>>> [0] drivers/usb/gadget/function/f_fs.c::ffs_user_copy_worker()
>>> 
>>> Sebastian
>> I think we need an entirely new API:
>> 
>> user_mm_ctx_t ctx = user_mm_ctx_get();
>> 
>> ...
>> 
>> use_user_mm_ctx(ctx);
>> unuse_user_mm_ctx(ctx);
>> 
>> ...
>> 
>> user_mm_ctx_put(ctx);
>> 
>> and ctx will store a copy of mm and PKRU.
>> 
> 
> That looks like a good API in general.  The ffs_user_copy_worker that
> Sebastian mentioned seems to be used by AIO, in which case of course it
> has to happen in a kernel thread.
> 
> But while the API is good, deciding on the desired semantics is
> "interesting".  The submitting thread might be changing PKRU between the
> time the I/O operation is submitted and the time it is completed, for
> example.

I think thereâs only one sensible answer: capture PKRU at the time of submission.