Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a configurable timer

[Andy Lutomirski]

On Jun 12, 2015 1:43 AM, "Borislav Petkov" <bp@xxxxxxx> wrote:
>
> On Tue, Jun 09, 2015 at 09:46:52AM -0700, Andy Lutomirski wrote:
> > I don't like this hack.  The compiler is entirely within is rights to
> > poke addr's cacheline (i.e. the stack) between the two instructions.
> > I'd suggest either making the thing a full cacheline long or using a
> > single asm statement.
>
> How about this:
>
>         /*
>          * This should be a memory location in a cache line which is
>          * unlikely to be touched by other processors.  The actual
>          * content is immaterial as it is not actually modified in any way.
>          */
>         mwait_ptr = &current_thread_info()->flags;
>
> and then
>
>         __monitor(mwait_ptr, 0, 0);
>
> We already do this in mwait_play_dead().
>
> However, am I even correct in assuming that ->flags won't really be
> touched as we're doing delay() and nothing pokes into current anyway?

We poke flags remotely, but not frequently enough for this to be a
problem.  However, I don't know that touching current in udelay is
okay.

How about some read-mostly percpu variable, such as cpu_tss?

--Andy
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