Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 21 Dec 2015 09:10:52 +0200 | From | "Michael S. Tsirkin" <> | Subject | Re: [Xen-devel] new barrier type for paravirt (was Re: [PATCH] virtio_ring: use smp_store_mb) |
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On Sun, Dec 20, 2015 at 08:59:44PM +0100, Peter Zijlstra wrote: > 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.
One of the uses for virtio is to allow testing an existing kernel on kvm just by loading a module, and this will break this usecase.
> I don't see people going to allow emitting the LOCK prefix (and growing > the kernel text size) for UP kernels.
Thinking about this more, maybe __smp_*mb is a better set of names.
The nice thing about it is that we can then have generic code that does basically
#ifdef CONFIG_SMP #define smp_mb() __smp_mb() #else #define smp_mb() barrier() #endif
and reuse this on all architectures.
So instead of a maintainance burden, we are actually removing code duplication.
-- MST
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