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

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Message-ID: <39ab2f2b-81bb-636e-933d-f3a5225aa020@intel.com>
Date:   Tue, 15 Jan 2019 12:54:49 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        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 1/15/19 12:26 PM, Andy Lutomirski wrote:
> I don't think we'd ever want kernel_fpu_end() to restore anything,
> right?  I'm a bit confused as to when this optimization would actually
> be useful.

Using AVX-512 as an example...

Let's say there was AVX-512 state, and a kernel_fpu_begin() user only
used AVX2.  We could totally avoid doing *any* AVX-512 state save/restore.

The init optimization doesn't help us if there _is_ AVX-512 state, and
the modified optimization only helps if we recently did a XRSTOR at
context switch and have not written to AVX-512 state since XRSTOR.

This probably only matters for AVX-512-using apps that have run on a
kernel with lots of kernel_fpu_begin()s that don't use AVX-512.  So, not
a big deal right now.