Re: [RFC PATCH 04/10] x86/fpu: eager switch PKRU state

[Rik van Riel]

On Wed, 2018-09-12 at 08:20 -0700, Andy Lutomirski wrote:
> > 
> > --- a/arch/x86/mm/pkeys.c
> > +++ b/arch/x86/mm/pkeys.c
> > @@ -18,6 +18,20 @@
> > 
> > #include <asm/cpufeature.h>             /* boot_cpu_has,
> > ...            */
> > #include <asm/mmu_context.h>            /*
> > vma_pkey()                   */
> > +#include <asm/fpu/internal.h>
> > +
> > +void write_pkru(u32 pkru)
> > +{
> > +    if (!boot_cpu_has(X86_FEATURE_OSPKE))
> > +        return;
> > +
> > +    current->thread.fpu.pkru = pkru;
> > +
> 
> I thought that the offset of PKRU in the xstate was fixed after boot.

You are right, it is.  However, that offset would need
to be stored somewhere, and the value read every time
we wanted to read or store the PKRU value from/to the
floating point state.

I suspect that would not be any faster than keeping a
copy of the PKRU value in a known location.

> Anyway, as written, this needs a lockdep assertion that weâre not
> preemptible, an explicit preempt_disable(), or a comment explaining
> why itâs okay if we get preempted in this function.
> 
> > +    __fpregs_changes_begin();

This handles the preemption disabling, see patch
3 of the series.

> > +    __fpregs_load_activate(&current->thread.fpu,
> > smp_processor_id());
> > +    __write_pkru(pkru);
> > +    __fpregs_changes_end();
> > +}
> > 
> > int __execute_only_pkey(struct mm_struct *mm)
> > {
> > -- 
> > 2.19.0
> > 
> 
> 
-- 
All Rights Reversed.

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