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Date:   Tue, 15 Jan 2019 14:15:16 +0100
From:   'Sebastian Andrzej Siewior' <bigeasy@...utronix.de>
To:     David Laight <David.Laight@...LAB.COM>
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

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?

> 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 would like to see some benefit of this first before adding/adjusting
the API in a way which makes it possible do something to do wrong. That
said, one thing I would like to do is to get rid of irq_fpu_usable() so
code can use FPU registers and needs not to implement a fallback.

> 	David

Sebastian

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