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    SubjectRe: [Xen-devel] new barrier type for paravirt (was Re: [PATCH] virtio_ring: use smp_store_mb)
    On Mon, Dec 21, 2015 at 10:47:49AM +0000, David Vrabel wrote:
    > 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

    Guest kernel - yes. But it's only needed because you
    are running on an SMP host.

    --
    MST


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