similar to: [PATCH] virtio_ring: Update weak barriers to use dma_wmb/rmb

On Tue, Apr 07, 2015 at 05:47:42PM -0700, Alexander Duyck wrote: > This change makes it so that instead of using smp_wmb/rmb which varies > depending on the kernel configuration we can can use dma_wmb/rmb which for > most architectures should be equal to or slightly more strict than > smp_wmb/rmb. > > The advantage to this is that these barriers are available to uniprocessor

On 04/08/2015 01:42 AM, Michael S. Tsirkin wrote: > On Tue, Apr 07, 2015 at 05:47:42PM -0700, Alexander Duyck wrote: >> This change makes it so that instead of using smp_wmb/rmb which varies >> depending on the kernel configuration we can can use dma_wmb/rmb which for >> most architectures should be equal to or slightly more strict than >> smp_wmb/rmb. >> >>

On 04/08/2015 01:42 AM, Michael S. Tsirkin wrote: > On Tue, Apr 07, 2015 at 05:47:42PM -0700, Alexander Duyck wrote: >> This change makes it so that instead of using smp_wmb/rmb which varies >> depending on the kernel configuration we can can use dma_wmb/rmb which for >> most architectures should be equal to or slightly more strict than >> smp_wmb/rmb. >> >>

virtio is using barriers to order memory accesses, thus dma_wmb/rmb is a good match. Build-tested on x86: Before [mst at tuck linux]$ size drivers/virtio/virtio_ring.o text data bss dec hex filename 11392 820 0 12212 2fb4 drivers/virtio/virtio_ring.o After mst at tuck linux]$ size drivers/virtio/virtio_ring.o text data bss dec hex filename

virtio is using barriers to order memory accesses, thus dma_wmb/rmb is a good match. Build-tested on x86: Before [mst at tuck linux]$ size drivers/virtio/virtio_ring.o text data bss dec hex filename 11392 820 0 12212 2fb4 drivers/virtio/virtio_ring.o After mst at tuck linux]$ size drivers/virtio/virtio_ring.o text data bss dec hex filename

Changes since v1: - replaced my asm-generic patch with an equivalent patch already in tip - add wrappers with virt_ prefix for better code annotation, as suggested by David Miller - dropped XXX in patch names as this makes vger choke, Cc all relevant mailing lists on all patches (not personal email, as the list becomes too long then) I parked this in vhost tree for now, but the

Changes since v1: - replaced my asm-generic patch with an equivalent patch already in tip - add wrappers with virt_ prefix for better code annotation, as suggested by David Miller - dropped XXX in patch names as this makes vger choke, Cc all relevant mailing lists on all patches (not personal email, as the list becomes too long then) I parked this in vhost tree for now, but the

On Wed, Apr 08, 2015 at 07:41:49AM -0700, Alexander Duyck wrote: > > On 04/08/2015 01:42 AM, Michael S. Tsirkin wrote: > >On Tue, Apr 07, 2015 at 05:47:42PM -0700, Alexander Duyck wrote: > >>This change makes it so that instead of using smp_wmb/rmb which varies > >>depending on the kernel configuration we can can use dma_wmb/rmb which for > >>most

This is really trying to cleanup some virt code, as suggested by Peter, who said > You could of course go fix that instead of mutilating things into > sort-of functional state. This work is needed for virtio, so it's probably easiest to merge it through my tree - is this fine by everyone? Arnd, if you agree, could you ack this please? Note to arch maintainers: please don't

This is really trying to cleanup some virt code, as suggested by Peter, who said > You could of course go fix that instead of mutilating things into > sort-of functional state. This work is needed for virtio, so it's probably easiest to merge it through my tree - is this fine by everyone? Arnd, if you agree, could you ack this please? Note to arch maintainers: please don't

On Thu, Dec 17, 2015 at 12:22:22PM +0100, Peter Zijlstra wrote: > On Thu, Dec 17, 2015 at 12:32:53PM +0200, Michael S. Tsirkin wrote: > > Seems to give a speedup on my box but I'm less sure about this one. E.g. as > > xchng faster than mfence on all/most intel CPUs? Anyone has an opinion? > > Would help if you Cc people who would actually know this :-) Good point. Glad

On Thu, Dec 17, 2015 at 12:22:22PM +0100, Peter Zijlstra wrote: > On Thu, Dec 17, 2015 at 12:32:53PM +0200, Michael S. Tsirkin wrote: > > Seems to give a speedup on my box but I'm less sure about this one. E.g. as > > xchng faster than mfence on all/most intel CPUs? Anyone has an opinion? > > Would help if you Cc people who would actually know this :-) Good point. Glad

Changes since v2: - extended checkpatch tests for barriers, and added patches teaching it to warn about incorrect usage of barriers (__smp_xxx barriers are for use by asm-generic code only), should help prevent misuse by arch code to address comments by Russell King - patched more instances of xen to use virt_ barriers as suggested by Stefano Stabellini - implemented a 2 byte xchg on sh

Changes since v2: - extended checkpatch tests for barriers, and added patches teaching it to warn about incorrect usage of barriers (__smp_xxx barriers are for use by asm-generic code only), should help prevent misuse by arch code to address comments by Russell King - patched more instances of xen to use virt_ barriers as suggested by Stefano Stabellini - implemented a 2 byte xchg on sh

virtio ring entries have exactly the acquire/release semantics: - reading used index acquires a ring entry from host - updating the available index releases it to host Thus when using weak barriers and building for SMP (as most people do), smp_load_acquire and smp_store_release will do exactly the right thing to synchronize with the host. In fact, QEMU already uses

virtio ring entries have exactly the acquire/release semantics: - reading used index acquires a ring entry from host - updating the available index releases it to host Thus when using weak barriers and building for SMP (as most people do), smp_load_acquire and smp_store_release will do exactly the right thing to synchronize with the host. In fact, QEMU already uses

On Thu, Dec 17, 2015 at 11:52:38AM +0100, Peter Zijlstra wrote: > On Thu, Dec 17, 2015 at 12:32:53PM +0200, Michael S. Tsirkin wrote: > > +static inline void virtio_store_mb(bool weak_barriers, > > + __virtio16 *p, __virtio16 v) > > +{ > > +#ifdef CONFIG_SMP > > + if (weak_barriers) > > + smp_store_mb(*p, v); > > + else > > +#endif >

On Thu, Dec 17, 2015 at 11:52:38AM +0100, Peter Zijlstra wrote: > On Thu, Dec 17, 2015 at 12:32:53PM +0200, Michael S. Tsirkin wrote: > > +static inline void virtio_store_mb(bool weak_barriers, > > + __virtio16 *p, __virtio16 v) > > +{ > > +#ifdef CONFIG_SMP > > + if (weak_barriers) > > + smp_store_mb(*p, v); > > + else > > +#endif >

We need a full barrier after writing out event index, using smp_store_mb there seems better than open-coding. As usual, we need a wrapper to account for strong barriers/non smp. It's tempting to use this in vhost as well, for that, we'll need a variant of smp_store_mb that works on __user pointers. Signed-off-by: Michael S. Tsirkin <mst at redhat.com> --- Seems to give a speedup

We need a full barrier after writing out event index, using smp_store_mb there seems better than open-coding. As usual, we need a wrapper to account for strong barriers/non smp. It's tempting to use this in vhost as well, for that, we'll need a variant of smp_store_mb that works on __user pointers. Signed-off-by: Michael S. Tsirkin <mst at redhat.com> --- Seems to give a speedup