Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch

[Dave Hansen <dave.hansen@linux.intel.com>]

On 06/15/2018 01:33 PM, Jason A. Donenfeld wrote:
> On Fri, Jun 15, 2018 at 8:32 PM Andy Lutomirski <luto@kernel.org> wrote:
>> quite in the form you imagined.  The idea that we've tossed around is
>> to restore FPU state on return to user mode.  Roughly, we'd introduce
>> a new thread flag TIF_FPU_UNLOADED (name TBD).
>> prepare_exit_to_usermode() would notice this flag, copy the fpstate to
>> fpregs, and clear the flag.  (Or maybe exit_to_usermode_loop() -- No
>> one has quite thought it through, but I think it should be outside the
>> loop.)  We'd update all the FPU accessors to understand the flag.
> Yes! This is exactly what I was thinking. Then those calls to begin()
> and end() could be placed as close to the actual FPU usage as
> possible.

Andy, what was the specific concern about PKRU?  That we might do:

	kernel_fpu_begin(); <- Saves the first time
	something()
	kernel_fpu_end(); <- Does not XRSTOR

	copy_from_user(); <- Sees old PKRU, does the wrong thing

	prepare_exit_to_usermode(); <- Does the XRSTOR
	// only now does PKRU have the right value
	SYSRET/IRET

?

Does that *matter* unless something() modified PKRU?  We could just make
the rule that nobody is supposed to mess with it and that it's not
covered by kernel_fpu_begin/end() semantics.  We could even
theoretically enforce that in a debug environment if we watch its value.