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Date: Mon, 21 Dec 2015 09:10:52 +0200 From: "Michael S. Tsirkin" <mst@...hat.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Andrew Cooper <andrew.cooper3@...rix.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, Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.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 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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