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