Linux Virtualization - Oct 2007 - LFENCE instruction (was: [rfc][patch 3/3] x86: optimise barriers)

Nick Piggin <npiggin@suse.de> wrote:
>
> Also, for non-wb memory. I don't think the Intel document referenced
> says anything about this, but the AMD document says that loads can pass
> loads (page 8, rule b).
> 
> This is why our rmb() is still an lfence.
BTW, Xen (in particular, the code in drivers/xen) uses mb/rmb/wmb
instead of smp_mb/smp_rmb/smp_wmb when it accesses memory that's
shared with other Xen domains or the hypervisor.

The reason this is necessary is because even if a Xen domain is
UP the hypervisor might be SMP.

It would be nice if we can have these adopt the new SMP barriers
on x86 instead of the IO ones as they currently do.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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On Wed, Oct 17, 2007 at 01:51:17PM +0800, Herbert Xu
wrote:
> Nick Piggin <npiggin@suse.de> wrote:
> >
> > Also, for non-wb memory. I don't think the Intel document
referenced
> > says anything about this, but the AMD document says that loads can
pass
> > loads (page 8, rule b).
> > 
> > This is why our rmb() is still an lfence.
> 
> BTW, Xen (in particular, the code in drivers/xen) uses mb/rmb/wmb
> instead of smp_mb/smp_rmb/smp_wmb when it accesses memory that's
> shared with other Xen domains or the hypervisor.
> 
> The reason this is necessary is because even if a Xen domain is
> UP the hypervisor might be SMP.
> 
> It would be nice if we can have these adopt the new SMP barriers
> on x86 instead of the IO ones as they currently do.
That's a good point actually. Something like raw_smp_*mb, which
always orders memory, but only for regular WB operatoins. I could
put that on the todo list...