On Mon, Jan 26, 2015 at 12:59:29PM -0700, Ross Zwisler wrote:
> This is interesting! I guess I'm confused as to how this solves the ordering
> issue, though. The "m" input vs "+m" output parameter will tell gcc whether
> or not the assembly can be reordered at compile time with respect to reads at
> that same location, correct?
>
> So if we have an inline function that could either read or write from gcc's
> point of view (input vs output parameter, depending on the branch), it seems
> like it would be forced to fall back to the most restrictive case (assume it
> will write), and not reorder with respect to reads. If so, you'd end up in
> the same place as using "+m" output, only now you've got an additional branch
> instead of a 3-way alternative.
>
> Am I misunderstanding this?
No, you're not, that is the right question. I was simply hypothesizing
about how we could do what hpa suggests but I don't have any other ideas
about having an "m" and an "+m" in the same inline asm statement.
My hunch is, the moment we have an "+m", the reordering would be
suppressed and that would not give us the CLWB case where we don't have
to suppress reordering wrt reads.
> Ah, yep, I definitely need to include an example flow in my commit comments.
> :) Here's a snip from my reply to hpa, to save searching:
>
> Both the flushes (wmb/clflushopt/clflush) and the pcommit are ordered
> by either mfence or sfence.
>
> An example function that flushes and commits a buffer could look like
> this (based on clflush_cache_range):
>
> void flush_and_commit_buffer(void *vaddr, unsigned int size)
> {
> void *vend = vaddr + size - 1;
>
> for (; vaddr < vend; vaddr += boot_cpu_data.x86_clflush_size)
> clwb(vaddr);
>
> /* Flush any possible final partial cacheline */
> clwb(vend);
>
> /*
> * sfence to order clwb/clflushopt/clflush cache flushes
> * mfence via mb() also works
> */
> wmb();
>
> pcommit();
Oh, so you need an SFENCE to flush out the preceding in-flight writes
*and* PCOMMIT for the persistent memory ranges. Ok, makes sense, PCOMMIT
deals with the persistent stores.
> /*
> * sfence to order pcommit
> * mfence via mb() also works
> */
> wmb();
Doc says PCOMMIT is not ordered wrt loads and SFENCE too. Don't we want
to be absolutely conservative here and use MFENCE both times?
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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