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

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Date:	Mon, 21 Dec 2015 10:47:49 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	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>,
	Ian Campbell <ian.campbell@...rix.com>
Subject: Re: [Xen-devel] new barrier type for paravirt (was Re: [PATCH] virtio_ring:
 use smp_store_mb)

On 20/12/15 09:25, Michael S. Tsirkin wrote:
> 
> I noticed that drivers/xen/xenbus/xenbus_comms.c uses
> full memory barriers to communicate with the other side.
> For example:
> 
>                 /* Must write data /after/ reading the consumer index.  * */
>                 mb();
> 
>                 memcpy(dst, data, avail);
>                 data += avail;
>                 len -= avail;
>         
>                 /* Other side must not see new producer until data is * there. */
>                 wmb();
>                 intf->req_prod += avail;
>                 
>                 /* Implies mb(): other side will see the updated producer. */
>                 notify_remote_via_evtchn(xen_store_evtchn);
> 
> To me, it looks like for guests compiled with CONFIG_SMP, smp_wmb and smp_mb
> would be sufficient, so mb() and wmb() here are only needed if
> a non-SMP guest runs on an SMP host.
> 
> Is my analysis correct?

For x86, yes.

For arm/arm64 I think so, but would prefer one of the Xen arm
maintainers to confirm.  In particular, whether inner-shareable barriers
are sufficient for memory shared with the hypervisor.

> So what I'm suggesting is something like the below patch,
> except instead of using virtio directly, a new set of barriers
> that behaves identically for SMP and non-SMP guests will be introduced.
> 
> And of course the weak barriers flag is not needed for Xen -
> that's a virtio only thing.
> 
> For example:
> 
> smp_pv_wmb()
> smp_pv_rmb()
> smp_pv_mb()

The smp_ prefix doesn't make a lot of sense to me here since these
barriers are going to be the same whether the kernel is SMP or not.

David
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