On Fri, May 13, 2016 at 05:46:17PM +0800, Jike Song wrote: > On 05/13/2016 04:12 AM, Neo Jia wrote: > > On Thu, May 12, 2016 at 01:05:52PM -0600, Alex Williamson wrote: > >> > >> If you're trying to equate the scale of what we need to track vs what > >> type1 currently tracks, they're significantly different. Possible > >> things we need to track include the pfn, the iova, and possibly a > >> reference count or some sort of pinned page map. In the pin-all model > >> we can assume that every page is pinned on map and unpinned on unmap, > >> so a reference count or map is unnecessary. We can also assume that we > >> can always regenerate the pfn with get_user_pages() from the vaddr, so > >> we don't need to track that. > > > > Hi Alex, > > > > Thanks for pointing this out, we will not track those in our next rev and > > get_user_pages will be used from the vaddr as you suggested to handle the > > single VM with both passthru + mediated device case. > > > > Just a gut feeling: > > Calling GUP every time for a particular vaddr, means locking mm->mmap_sem > every time for a particular process. If the VM has dozens of VCPU, which > is not rare, the semaphore is likely to be the bottleneck. Hi Jike, We do need to hold the lock of mm->mmap_sem for the VMM/QEMU process, but I don't quite follow the reasoning with "dozens of vcpus", one situation that I can think of is that we have other thread competing with the mmap_sem for the VMM/QEMU process within KVM kernel such as hva_to_pfn, after a quick search it seems only mostly gets used by iotcl "KVM_ASSIGN_PCI_DEVICE". We will definitely conduct performance analysis with large configuration on servers with E5-2697 v4. :-) Thanks, Neo > > > -- > Thanks, > Jike > -- 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
