On Thu, 18 Jun 2015 16:47:33 +0200 "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > On Thu, Jun 18, 2015 at 03:46:14PM +0200, Paolo Bonzini wrote: > > > > > > On 18/06/2015 15:19, Michael S. Tsirkin wrote: > > > On Thu, Jun 18, 2015 at 01:50:32PM +0200, Paolo Bonzini wrote: > > >> > > >> > > >> On 18/06/2015 13:41, Michael S. Tsirkin wrote: > > >>> On Thu, Jun 18, 2015 at 01:39:12PM +0200, Igor Mammedov wrote: > > >>>> Lets leave decision upto users instead of making them live with > > >>>> crashing guests. > > >>> > > >>> Come on, let's fix it in userspace. > > >> > > >> It's not trivial to fix it in userspace. Since QEMU uses RCU there > > >> isn't a single memory map to use for a linear gpa->hva map. > > > > > > Could you elaborate? > > > > > > I'm confused by this mention of RCU. > > > You use RCU for accesses to the memory map, correct? > > > So memory map itself is a write side operation, as such all you need to > > > do is take some kind of lock to prevent conflicting with other memory > > > maps, do rcu sync under this lock. > > > > You're right, the problem isn't directly related to RCU. RCU would be > > easy to handle by using synchronize_rcu instead of call_rcu. While I > > identified an RCU-related problem with Igor's patches, it's much more > > entrenched. > > > > RAM can be used by asynchronous operations while the VM runs, between > > address_space_map and address_space_unmap. It is possible and common to > > have a quiescent state between the map and unmap, and a memory map > > change can happen in the middle of this. Normally this is not a > > problem, because changes to the memory map do not make the hva go away > > (memory regions are reference counted). > > Right, so you want mmap(MAP_NORESERVE) when that reference > count becomes 0. > > > However, with Igor's patches a memory_region_del_subregion will cause a > > mmap(MAP_NORESERVE), which _does_ have the effect of making the hva go away. > > > > I guess one way to do it would be to alias the same page in two places, > > one for use by vhost and one for use by everything else. However, the > > kernel does not provide the means to do this kind of aliasing for > > anonymous mmaps. > > > > Paolo > > Basically pages go away on munmap, so won't simple > lock > munmap > mmap(MAP_NORESERVE) > unlock > do the trick? at what time are you suggesting to do this? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
