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

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Date:	Fri, 13 Mar 2015 14:03:13 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Borislav Petkov <bp@...en8.de>, 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()

On Thu, 2015-03-12 at 11:58 +0100, Ingo Molnar wrote:
> > > +/**
> > > + * 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.

Sure, will do.

> > > + * 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'.

The original version, copied in part from clflush_cache_range, did do
everything with void* pointers.  I changed it to use char* pointers based on
feedback from hpa.  :)

It seems like both have arguments for them.  Char pointer arithmetic has the
advantage that its behavior is standard in C, so it's not specific to gcc.  I
agree that void* has the advantage that it fits more naturally with the types
of the parameters passed in, requiring no casting.

I honestly don't feel strongly either way - please let me know what you guys
prefer in the x86 arch code.

> > > + *         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!

Yep.  C++ style // comments are happily accepted by gcc in C code, though, and
this was my attempt to get around the fact that /* */ style comments can't be
nested.  I couldn't think of a more elegant way of having code + comments in a
kerneldoc comment.  I agree that if this code were ever to be pulled out and
used, the comment style would need to be corrected to be the standard kernel
style.

> 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

Sure.

Thanks,
- Ross


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