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

[Rik van Riel <riel@surriel.com>]

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On Fri, 2018-06-15 at 13:42 -0700, Dave Hansen wrote:
> 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.

KVM needs to change out guest and host PKRU values
when switching between guest and host mode, but
since f775b13eedee ("x86,kvm: move qemu/guest FPU 
switching out to vcpu_run") that no longer happens
under kernel_fpu_begin/end so we don't need to care
about that :)

-- 
All Rights Reversed.

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