Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until switch to userspace
From: Andy Lutomirski
Date: Mon Oct 03 2016 - 23:03:21 EST
On Mon, Oct 3, 2016 at 7:47 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> On Mon, 2016-10-03 at 19:09 -0700, Andy Lutomirski wrote:
>> > Having two separate status booleans for "registers valid"
>> > and "memory valid" may make more sense.
>> I have no problem with the concept of "owner_ctx", and I think it's a
>> perfectly reasonable data structure. My problem with it is that it's
>> subtle and knowledge of it is spread all over the place. Just going
>> with "registers valid" in a variable won't work, I think, because
>> there's nowhere to put it. We need to be able to delete a struct fpu
>> while that struct fpu might have a valid copy in a different cpu's
>> Anyway, feel free to tell me that I'm making this too difficult :)
> How about we rename fpu_want_lazy_restore to
> fpu_registers_valid()? Problem solved :)
> Then we can rename __cpu_disable_lazy_restore
> to fpu_invalidate_registers(), and call that
> before we modify any in-memory FPU state.
Sounds good to me.
>> > We can get rid of fpu.counter, since nobody uses it
>> > any more.
>> We should definitely do this.
>> Maybe getting in some cleanups first (my lazy fpu deletion,
>> fpu.counter removal, etc) first is the way to go.
> Sounds good. I will keep my patch 1/4 as part of the
> cleanup series, and will not move on to the harder
> stuff until after the cleanups.
> Any other stuff I should clean up while we're there?
Almost certainly, but nothing I'm thinking of right now :)
>> > > > >
>> > You are right, read_pkru() and write_pkru() can only deal with
>> > the pkru state being present in registers. Is this because of an
>> > assumption in the code, or because of a hardware requirement?
> read_pkru and write_pkru would be candidates for using
> fpu_registers_valid, and potentially a fpu_make_registers_valid,
> which restores the contents of the fpu registers from memory,
> if fpu_registers_valid is not true.
> Likewise, we can have an fpu_make_memory_valid to ensure the
> in kernel memory copy of the FPU registers is valid, potentially
> a _read and _write version that do exactly what the pstate code
> wants today.
> Would that make sense as an API?
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