On Tue, Mar 12, 2019 at 10:52:15AM +0800, Jason Wang wrote: > > On 2019/3/11 下午8:48, Michael S. Tsirkin wrote: > > On Mon, Mar 11, 2019 at 03:40:31PM +0800, Jason Wang wrote: > > > On 2019/3/9 上午3:48, Andrea Arcangeli wrote: > > > > Hello Jeson, > > > > > > > > On Fri, Mar 08, 2019 at 04:50:36PM +0800, Jason Wang wrote: > > > > > Just to make sure I understand here. For boosting through huge TLB, do > > > > > you mean we can do that in the future (e.g by mapping more userspace > > > > > pages to kenrel) or it can be done by this series (only about three 4K > > > > > pages were vmapped per virtqueue)? > > > > When I answered about the advantages of mmu notifier and I mentioned > > > > guaranteed 2m/gigapages where available, I overlooked the detail you > > > > were using vmap instead of kmap. So with vmap you're actually doing > > > > the opposite, it slows down the access because it will always use a 4k > > > > TLB even if QEMU runs on THP or gigapages hugetlbfs. > > > > > > > > If there's just one page (or a few pages) in each vmap there's no need > > > > of vmap, the linearity vmap provides doesn't pay off in such > > > > case. > > > > > > > > So likely there's further room for improvement here that you can > > > > achieve in the current series by just dropping vmap/vunmap. > > > > > > > > You can just use kmap (or kmap_atomic if you're in preemptible > > > > section, should work from bh/irq). > > > > > > > > In short the mmu notifier to invalidate only sets a "struct page * > > > > userringpage" pointer to NULL without calls to vunmap. > > > > > > > > In all cases immediately after gup_fast returns you can always call > > > > put_page immediately (which explains why I'd like an option to drop > > > > FOLL_GET from gup_fast to speed it up). > > > > > > > > Then you can check the sequence_counter and inc/dec counter increased > > > > by _start/_end. That will tell you if the page you got and you called > > > > put_page to immediately unpin it or even to free it, cannot go away > > > > under you until the invalidate is called. > > > > > > > > If sequence counters and counter tells that gup_fast raced with anyt > > > > mmu notifier invalidate you can just repeat gup_fast. Otherwise you're > > > > done, the page cannot go away under you, the host virtual to host > > > > physical mapping cannot change either. And the page is not pinned > > > > either. So you can just set the "struct page * userringpage = page" > > > > where "page" was the one setup by gup_fast. > > > > > > > > When later the invalidate runs, you can just call set_page_dirty if > > > > gup_fast was called with "write = 1" and then you clear the pointer > > > > "userringpage = NULL". > > > > > > > > When you need to read/write to the memory > > > > kmap/kmap_atomic(userringpage) should work. > > > Yes, I've considered kmap() from the start. The reason I don't do that is > > > large virtqueue may need more than one page so VA might not be contiguous. > > > But this is probably not a big issue which just need more tricks in the > > > vhost memory accessors. > > > > > > > > > > In short because there's no hardware involvement here, the established > > > > mapping is just the pointer to the page, there is no need of setting > > > > up any pagetables or to do any TLB flushes (except on 32bit archs if > > > > the page is above the direct mapping but it never happens on 64bit > > > > archs). > > > I see, I believe we don't care much about the performance of 32bit archs (or > > > we can just fallback to copy_to_user() friends). > > Using copyXuser is better I guess. > > > Ok. > > > > > > > Using direct mapping (I > > > guess kernel will always try hugepage for that?) should be better and we can > > > even use it for the data transfer not only for the metadata. > > > > > > Thanks > > We can't really. The big issue is get user pages. Doing that on data > > path will be slower than copyXuser. > > > I meant if we can find a way to avoid doing gup in datapath. E.g vhost > maintain a range tree and add or remove ranges through MMU notifier. Then in > datapath, if we find the range, then use direct mapping otherwise > copy_to_user(). > > Thanks We can try. But I'm not sure there's any reason to think there's any locality there. > > > Or maybe it won't with the > > amount of mitigations spread around. Go ahead and try. > > > >
