linux-kernel - Re: [PATCH 03/17] fpga: dfl: fme: support 512bit data width PR

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Date:   Wed, 27 Mar 2019 12:37:43 +0800
From:   Wu Hao <hao.wu@...el.com>
To:     Scott Wood <swood@...hat.com>
Cc:     Alan Tull <atull@...nel.org>, Moritz Fischer <mdf@...nel.org>,
        linux-fpga@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, Ananda Ravuri <ananda.ravuri@...el.com>,
        Xu Yilun <yilun.xu@...el.com>
Subject: Re: [PATCH 03/17] fpga: dfl: fme: support 512bit data width PR

On Tue, Mar 26, 2019 at 04:22:34PM -0500, Scott Wood wrote:
> On Tue, 2019-03-26 at 14:33 -0500, Alan Tull wrote:
> > On Mon, Mar 25, 2019 at 5:58 PM Scott Wood <swood@...hat.com> wrote:
> > 
> > Hi Scott,
> > 
> > > On Mon, 2019-03-25 at 17:53 -0500, Scott Wood wrote:
> > > > On Mon, 2019-03-25 at 11:07 +0800, Wu Hao wrote:
> > > > > +#if defined(CONFIG_X86) && defined(CONFIG_AS_AVX512)
> > > > > +
> > > > > +#include <asm/fpu/api.h>
> > > > > +
> > > > > +static inline void copy512(void *src, void __iomem *dst)
> > > > > +{
> > > > > +   kernel_fpu_begin();
> > > > > +
> > > > > +   asm volatile("vmovdqu64 (%0), %%zmm0;"
> > > > > +                "vmovntdq %%zmm0, (%1);"
> > > > > +                :
> > > > > +                : "r"(src), "r"(dst));
> > > > > +
> > > > > +   kernel_fpu_end();
> > > > > +}
> > > > 
> > > > Shouldn't there be some sort of check that AVX512 is actually
> > > > supported
> > > > on the running system?
> > > > 
> > > > Also, src should be const, and the asm statement should have a memory
> > > > clobber.

Yes, I will fix this in the next version.

> > > > 
> > > > > +#else
> > > > > +static inline void copy512(void *src, void __iomem *dst)
> > > > > +{
> > > > > +   WARN_ON_ONCE(1);
> > > > > +}
> > > > > +#endif
> > > > 
> > > > Likewise, this will be called if a revision 2 device is used on non-
> > > > x86
> > > > (or on x86 with an old binutils).  The driver should fall back to 32-
> > > > bit
> > > > in such cases.

Unfortunately revision 2 is only for integrated FPGA solution, and it doesn't
support any fallback solution (original 32bit data partial reconfiguration is
not supported any more), so driver has to WARN in such path.

> > > 
> > > Sorry, I missed the comment about revision 2 only being on integrated
> > > devices -- but will that always be the case?  Seems worthwhile to check
> > > for
> > > AVX512 support anyway.  And there's still the possibility of being built
> > > with an old binutils such that CONFIG_AS_AVX512 is not set, or running
> > > on a
> > > kernel where avx512 was disabled via a boot option.
> > 
> > The code checks for CONFIG_AS_AVX512 above.
> 
> That just indicates that binutils supports it.  Plus, the code does not
> check for CONFIG_AS_AVX512 when deciding whether to set pr_datawidth to 64
> (and thus call copy512), so you'll get a WARN_ON rather than falling back to
> 32-bit.
> 
> > What boot option are you referring to?
> 
> clearcpuid=304

Just tried it, my system was down after running above AVX512 with this option.

I agree that it needs to add some check code to make sure it's safe to run
such instructions. I will add some cpu_feature_enabled() check in the next
version.

Thanks a lot for the review and comments.

Hao

> 
> -Scott
>