linux-kernel - Re: [PATCH v3 01/12] fpga: dfl: fme: support 512bit data width PR

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Date:   Wed, 14 Aug 2019 11:34:15 -0500
From:   Scott Wood <swood@...hat.com>
To:     Wu Hao <hao.wu@...el.com>, Greg KH <gregkh@...uxfoundation.org>
Cc:     mdf@...nel.org, linux-fpga@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, atull@...nel.org,
        Ananda Ravuri <ananda.ravuri@...el.com>,
        Xu Yilun <yilun.xu@...el.com>
Subject: Re: [PATCH v3 01/12] fpga: dfl: fme: support 512bit data width PR

On Wed, 2019-07-24 at 22:22 +0800, Wu Hao wrote:
> On Wed, Jul 24, 2019 at 11:35:32AM +0200, Greg KH wrote:
> > On Tue, Jul 23, 2019 at 12:51:24PM +0800, Wu Hao wrote:
> > >  
> > > @@ -67,8 +69,43 @@
> > >  #define PR_WAIT_TIMEOUT   8000000
> > >  #define PR_HOST_STATUS_IDLE	0
> > >  
> > > +#if defined(CONFIG_X86) && defined(CONFIG_AS_AVX512)
> > > +
> > > +#include <linux/cpufeature.h>
> > > +#include <asm/fpu/api.h>
> > > +
> > > +static inline int is_cpu_avx512_enabled(void)
> > > +{
> > > +	return cpu_feature_enabled(X86_FEATURE_AVX512F);
> > > +}
> > 
> > That's a very arch specific function, why would a driver ever care about
> > this?
> 
> Yes, this is only applied to a specific FPGA solution, which FPGA
> has been integrated with XEON. Hardware indicates this using register
> to software. As it's cpu integrated solution, so CPU always has this
> AVX512 capability. The only check we do, is make sure this is not
> manually disabled by kernel.
> 
> With this hardware, software could use AVX512 to accelerate the FPGA
> partial reconfiguration as mentioned in the patch commit message.
> It brings performance benifits to people who uses it. This is only one
> optimization (512 vs 32bit data write to hw) for a specific hardware.

I thought earlier you said that 512 bit accesses were required for this
particular integrated-only version of the device, and not just an
optimization?

> > > +#else
> > > +static inline int is_cpu_avx512_enabled(void)
> > > +{
> > > +	return 0;
> > > +}
> > > +
> > > +static inline void copy512(const void *src, void __iomem *dst)
> > > +{
> > > +	WARN_ON_ONCE(1);
> > 
> > Are you trying to get reports from syzbot?  :)
> 
> Oh.. no.. I will remove it. :)
> 
> Thank you very much!

What's wrong with this?  The driver should never call copy512() if
is_cpu_avx512_enabled() returns 0, and if syzbot can somehow make the driver
do so, then yes we do want a report.

-Scott