linux-kernel - RE: [PATCH] drm/i915: check to see if SIMD registers are available before using SIMD

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Message-ID: <21391cdac137449ab82c1fb5444eeb66@AcuMS.aculab.com>
Date:   Fri, 1 May 2020 11:34:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sebastian Andrzej Siewior' <bigeasy@...utronix.de>,
        "Jason A. Donenfeld" <Jason@...c4.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "chris@...is-wilson.co.uk" <chris@...is-wilson.co.uk>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] drm/i915: check to see if SIMD registers are available
 before using SIMD

From: Sebastian Andrzej Siewior
> Sent: 01 May 2020 11:42
> On 2020-04-30 16:10:16 [-0600], Jason A. Donenfeld wrote:
> > Sometimes it's not okay to use SIMD registers, the conditions for which
> > have changed subtly from kernel release to kernel release. Usually the
> > pattern is to check for may_use_simd() and then fallback to using
> > something slower in the unlikely case SIMD registers aren't available.
> > So, this patch fixes up i915's accelerated memcpy routines to fallback
> > to boring memcpy if may_use_simd() is false.
> 
> That would indicate that these functions are used from IRQ/softirq which
> break otherwise if the kernel is also using the registers. The crypto
> code uses it for that purpose.
> 
> So
>    Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> 
> May I ask how large the memcpy can be? I'm asking in case it is large
> and an explicit rescheduling point might be needed.

It is also quite likely that a 'rep movs' copy will be at least just as
fast on modern hardware.

Clearly if you are copying to/from PCIe memory you need the largest
resisters possible - but I think the graphics buffers are mapped cached?
(Otherwise I wouldn't see 3ms 'spins' while it invalidates the
entire screen buffer cache.)

	David

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