linux-kernel - Re: [PATCH 23/27] x86/fpu: Defer FPU state load until return to userspace

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Message-ID: <20190415091445.el2tj3yg6j3kfuom@linutronix.de>
Date:   Mon, 15 Apr 2019 11:14:45 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 23/27] x86/fpu: Defer FPU state load until return to
 userspace

On 2019-04-12 19:29:31 [+0200], Borislav Petkov wrote:
> > - in kernel_fpu_begin() we may trash the task's FPU state (by saving its
> >   registers or by resetting fpu_fpregs_owner_ctx).
> 
> Do we care?
> 
> You mean, in case you have workloads which might involve a lot of
> in-kernel FPU use which would punish task context switches?

Well, depends how you look on it. So for instance:
- Does kernel_fpu_begin() save task's FPU registers or are they already
  stored and it is a nop.
- The task avoid loading its registers on return to userland but that
  FPU usage in BH made it impossible.
- Is there any FPU usage in BH?

And there is this: My program usually works but with this scheduling
pattern and FPU usage in kernel it computes wrong result.
 
Sebastian