linux-kernel - Re: [Xen-devel] new barrier type for paravirt (was Re: [PATCH] virtio_ring: use smp_store

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Message-ID: <20151220195944.GT6344@twins.programming.kicks-ass.net>
Date:	Sun, 20 Dec 2015 20:59:44 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Cooper <andrew.cooper3@...rix.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>, qemu-devel@...gnu.org,
	Alexander Duyck <alexander.duyck@...il.com>,
	linux-kernel@...r.kernel.org,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [Xen-devel] new barrier type for paravirt (was Re: [PATCH]
 virtio_ring: use smp_store_mb)

On Sun, Dec 20, 2015 at 05:07:19PM +0000, Andrew Cooper wrote:
> 
> Very much +1 for fixing this.
> 
> Those names would be fine, but they do add yet another set of options in
> an already-complicated area.
> 
> An alternative might be to have the regular smp_{w,r,}mb() not revert
> back to nops if CONFIG_PARAVIRT, or perhaps if pvops have detected a
> non-native environment.  (I don't know how feasible this suggestion is,
> however.)

So a regular SMP kernel emits the LOCK prefix and will patch it out with
a DS prefix (iirc) when it finds but a single CPU. So for those you
could easily do this.

However an UP kernel will not emit the LOCK and do no patching.

So if you're willing to make CONFIG_PARAVIRT depend on CONFIG_SMP or
similar, this is doable.

I don't see people going to allow emitting the LOCK prefix (and growing
the kernel text size) for UP kernels.
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