linux-kernel - RE: [PATCH v6] x86: load FPU registers on return to userland

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Date:   Tue, 15 Jan 2019 14:33:32 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sebastian Andrzej Siewior' <bigeasy@...utronix.de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "kvm@...r.kernel.org" <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 v6] x86: load FPU registers on return to userland

From: 'Sebastian Andrzej Siewior'
> Sent: 15 January 2019 13:15
> On 2019-01-15 12:44:53 [+0000], David Laight wrote:
> > Once this is done it might be worth while adding a parameter to
> > kernel_fpu_begin() to request the registers only when they don't
> > need saving.
> > This would benefit code paths where the gains are reasonable but not massive.
> 
> So if saving + FPU code is a small win why not do this always?

I was thinking of the case when the cost of the fpu save is greater
than the saving.
This might be true for (say) a crc on a short buffer.

> > The return value from kernel_fpu_begin() ought to indicate which
> > registers are available - none, SSE, SSE2, AVX, AVX512 etc.
> > So code can use an appropriate implementation.
> > (I've not looked to see if this is already the case!)
> 
> Either everything is saved or nothing. So if SSE registers are saved
> then AVX512 are, too.

(I know that - I've written fpu save code for AVX).
I was thinking that the return value would depend on what the cpu supports.

In fact, given some talk about big-little cpus it might be worth being
able to ask for a specific register set.
Potentially that could cause a processor switch.

	David

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