Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until switch to userspace

[Andy Lutomirski <luto@amacapital.net>]

On Oct 3, 2016 7:11 PM, "Rik van Riel" <riel@redhat.com> wrote:
>
> On Mon, 2016-10-03 at 14:36 -0700, Andy Lutomirski wrote:
> >
> > Anything else that tries to read task xstate from memory, i.e. MPX
> > and
> > PKRU.  (Although if we switch to eager-switched PKRU, then PKRU stops
> > mattering for this purpose.)
> >
> > Actually, I don't see any way your patches can be compatible with
> > PKRU
> > without switching to eager-switched PKRU.
>
> There is one case where the in-register PKRU state matters:
> - user space accesses to memory
>
> There are several cases where the in-memory PKRU state would
> suffice:
> - get_user_pages(_fast) to the local task (could also use registers)
> - setting VMA/PTE permission bits (could also use registers)
>
> There is one case where only in-memory PKRU state works, where
> PKRU is currently simply ignored:
> - get_user_pages to another task's memory

Also __get_user, etc.  I don't think you want to start playing with
TIF_LOAD_FPU there.  I think tracking PKRU separately and eagerly
loading it with WRPKRU may be the only decent choice here.

>
> Dave, are there major obstacles to making read_pkru and write_pkru
> work with in-memory state?
>
> Would it be better for read/write_pkru to force the FPU state
> to get loaded into registers, under the assumption that if things
> like get_user_pages_fast happens, we will likely switch back to
> userspace soon, anyway?
>
> Would that assumption be wrong with KVM? :)

>
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