linux-kernel - Re: [PATCH] x86: Add kerneldoc for pcommit

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Date:	Thu, 12 Mar 2015 11:58:43 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	H Peter Anvin <h.peter.anvin@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: Add kerneldoc for pcommit_sfence()


* Borislav Petkov <bp@...en8.de> wrote:

> On Wed, Mar 11, 2015 at 12:19:21PM -0600, Ross Zwisler wrote:
> > Add kerneldoc comments for pcommit_sfence() describing the purpose of
> > the pcommit instruction and demonstrating the usage of that instruction.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > Cc: H Peter Anvin <h.peter.anvin@...el.com>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Borislav Petkov <bp@...en8.de>
> > ---
> >  arch/x86/include/asm/special_insns.h | 37 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 37 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> > index aeb4666e0c0a..1ae81757c05b 100644
> > --- a/arch/x86/include/asm/special_insns.h
> > +++ b/arch/x86/include/asm/special_insns.h
> > @@ -215,6 +215,43 @@ static inline void clwb(volatile void *__p)
> >  		: [pax] "a" (p));
> >  }
> >  
> > +/**
> > + * pcommit_sfence() - persistent commit and fence
> > + *
> > + * The pcommit instruction ensures that data that has been flushed from the
> > + * processor's cache hierarchy with clwb, clflushopt or clflush is accepted to
> > + * memory and is durable on the DIMM.  The primary use case for this is
> > + * persistent memory.

Please capitalize canonical instruction names like the CPU makers do, 
so that they stand out better in free flowing English text, i.e. 
something like:

 *
 * The PCOMMIT instruction ensures that data that has been flushed from the
 * processor's cache hierarchy with CLWB, CLFLUSHOPT or CLFLUSH is accepted to
 * memory and is durable on the DIMM.  The primary use case for this is
 * persistent memory.
 *

> > + * This function shows how to properly use clwb/clflushopt/clflush and pcommit
> > + * with appropriate fencing:

Ditto.

> > + *
> > + * void flush_and_commit_buffer(void *vaddr, unsigned int size)
> > + * {
> > + *         unsigned long clflush_mask = boot_cpu_data.x86_clflush_size - 1;
> > + *         char *vend = (char *)vaddr + size;

So here we cast vaddr to (char *) - which is unnecessary, as 'void *' 
has byte granular pointer arithmetics.

And 'vend' should be void *' to begin with, to match the type 
of 'vaddr'.

> > + *         char *p;

Ditto.

> > + *
> > + *         for (p = (char *)((unsigned long)vaddr & ~clflush_mask);
> > + *              p < vend; p += boot_cpu_data.x86_clflush_size)
> > + *                 clwb(p);
> > + *
> > + *         // sfence to order clwb/clflushopt/clflush cache flushes
> > + *         // mfence via mb() also works

Yeah so this isn't a C++ kernel, thank all the 3000+ gods and other 
supreme beings worshipped on this planet!

> > + *         wmb();
> > + *
> > + *         // pcommit and the required sfence for ordering

Ditto.

> > + *         pcommit_sfence();
> > + * }
> > + *
> > + * After this function completes the data pointed to by vaddr is has been
> 
> s/is //
> 
> I fixed it up while applying,

Also please put 'vaddr' into single quotes, to make the parameter name 
stand out better in written text:

> > + * After this function completes the data pointed to by 'vaddr' has been

Thanks,

	Ingo
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