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

From: Rahul Lakkireddy <[email-masked]>
Date: Tue, 20 Mar 2018 19:02:07 +0530

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 <[email-masked]>
> > Signed-off-by: Ganesh Goudar <[email-masked]>
> 
> 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.

> As far as I can tell the code where you are using this is a debug
> facility. What's the point? Debug is hardly a performance critical problem.
> 

On High Availability Server, the logs of the failing system must be
collected as quickly as possible.  So, we're concerned with the amount
of time taken to collect our large on-chip memory.  We see improvement
in doing 256-bit reads at a time.

Thanks,
Rahul