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? -- MST -- 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
