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SubjectRe: [RFC PATCH 04/10] x86/fpu: eager switch PKRU state
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> On Sep 12, 2018, at 8:30 AM, Rik van Riel <riel@surriel.com> wrote:
>
> 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 we’re not
>> preemptible, an explicit preempt_disable(), or a comment explaining
>> why it’s okay if we get preempted in this function.
>>
>>> + __fpregs_changes_begin();
>
> This handles the preemption disabling, see patch
> 3 of the series.

Sure, but the first write is *before* this. So we can be preempted with the two copies of PKRU being out of sync.

>
>>> + __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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