linux-kernel - Re: [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write

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Message-ID: <20180321122853.GC3245@chelsio.com>
Date:   Wed, 21 Mar 2018 17:58:54 +0530
From:   Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        Ganesh GR <ganeshgr@...lsio.com>,
        Nirranjan Kirubaharan <nirranjan@...lsio.com>,
        Indranil Choudhury <indranil@...lsio.com>
Subject: Re: [RFC PATCH 2/3] x86/io: implement 256-bit IO read and write

On Tuesday, March 03/20/18, 2018 at 20:12:15 +0530, Alexander Duyck wrote:
> On Tue, Mar 20, 2018 at 6:32 AM, Rahul Lakkireddy
> <rahul.lakkireddy@...lsio.com> wrote:
> > On Monday, March 03/19/18, 2018 at 20:13:10 +0530, Thomas Gleixner wrote:
> >> On Mon, 19 Mar 2018, Rahul Lakkireddy wrote:
> >>
> >> > Use VMOVDQU AVX CPU instruction when available to do 256-bit
> >> > IO read and write.
> >>
> >> That's not what the patch does. See below.
> >>
> >> > Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@...lsio.com>
> >> > Signed-off-by: Ganesh Goudar <ganeshgr@...lsio.com>
> >>
> >> That Signed-off-by chain is wrong....
> >>
> >> > +#ifdef CONFIG_AS_AVX
> >> > +#include <asm/fpu/api.h>
> >> > +
> >> > +static inline u256 __readqq(const volatile void __iomem *addr)
> >> > +{
> >> > +   u256 ret;
> >> > +
> >> > +   kernel_fpu_begin();
> >> > +   asm volatile("vmovdqu %0, %%ymm0" :
> >> > +                : "m" (*(volatile u256 __force *)addr));
> >> > +   asm volatile("vmovdqu %%ymm0, %0" : "=m" (ret));
> >> > +   kernel_fpu_end();
> >> > +   return ret;
> >>
> >> You _cannot_ assume that the instruction is available just because
> >> CONFIG_AS_AVX is set. The availability is determined by the runtime
> >> evaluated CPU feature flags, i.e. X86_FEATURE_AVX.
> >>
> >
> > Ok.  Will add boot_cpu_has(X86_FEATURE_AVX) check as well.
> >
> >> Aside of that I very much doubt that this is faster than 4 consecutive
> >> 64bit reads/writes as you have the full overhead of
> >> kernel_fpu_begin()/end() for each access.
> >>
> >> You did not provide any numbers for this so its even harder to
> >> determine.
> >>
> >
> > Sorry about that.  Here are the numbers with and without this series.
> >
> > When reading up to 2 GB on-chip memory via MMIO, the time taken:
> >
> > Without Series        With Series
> > (64-bit read)         (256-bit read)
> >
> > 52 seconds            26 seconds
> >
> > As can be seen, we see good improvement with doing 256-bits at a
> > time.
> 
> Instead of framing this as an enhanced version of the read/write ops
> why not look at replacing or extending something like the
> memcpy_fromio or memcpy_toio operations? It would probably be more
> comparable to what you are doing if you are wanting to move large
> chunks of memory from one region to another, and it should translate
> into something like AVX instructions once the CPU optimizations kick
> in for a memcpy.
> 

Ok. Will look into this approach.

Thanks,
Rahul