Messages in this thread | | | Subject | Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() | From | Jason Wang <> | Date | Tue, 12 Mar 2019 15:53:37 +0800 |
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On 2019/3/12 下午3:51, Jason Wang wrote: > > On 2019/3/12 下午1:14, James Bottomley wrote: >> On Tue, 2019-03-12 at 10:59 +0800, Jason Wang wrote: >>> On 2019/3/12 上午2:14, David Miller wrote: >>>> From: "Michael S. Tsirkin" <mst@redhat.com> >>>> Date: Mon, 11 Mar 2019 09:59:28 -0400 >>>> >>>>> On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote: >>>>>> On 2019/3/8 下午10:12, Christoph Hellwig wrote: >>>>>>> On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote: >>>>>>>> This series tries to access virtqueue metadata through >>>>>>>> kernel virtual >>>>>>>> address instead of copy_user() friends since they had too >>>>>>>> much >>>>>>>> overheads like checks, spec barriers or even hardware >>>>>>>> feature >>>>>>>> toggling. This is done through setup kernel address through >>>>>>>> vmap() and >>>>>>>> resigter MMU notifier for invalidation. >>>>>>>> >>>>>>>> Test shows about 24% improvement on TX PPS. TCP_STREAM >>>>>>>> doesn't see >>>>>>>> obvious improvement. >>>>>>> How is this going to work for CPUs with virtually tagged >>>>>>> caches? >>>>>> Anything different that you worry? >>>>> If caches have virtual tags then kernel and userspace view of >>>>> memory >>>>> might not be automatically in sync if they access memory >>>>> through different virtual addresses. You need to do things like >>>>> flush_cache_page, probably multiple times. >>>> "flush_dcache_page()" >>> >>> I get this. Then I think the current set_bit_to_user() is suspicious, >>> we >>> probably miss a flush_dcache_page() there: >>> >>> >>> static int set_bit_to_user(int nr, void __user *addr) >>> { >>> unsigned long log = (unsigned long)addr; >>> struct page *page; >>> void *base; >>> int bit = nr + (log % PAGE_SIZE) * 8; >>> int r; >>> >>> r = get_user_pages_fast(log, 1, 1, &page); >>> if (r < 0) >>> return r; >>> BUG_ON(r != 1); >>> base = kmap_atomic(page); >>> set_bit(bit, base); >>> kunmap_atomic(base); >> This sequence should be OK. get_user_pages() contains a flush which >> clears the cache above the user virtual address, so on kmap, the page >> is coherent at the new alias. On parisc at least, kunmap embodies a >> flush_dcache_page() which pushes any changes in the cache above the >> kernel virtual address back to main memory and makes it coherent again >> for the user alias to pick it up. > > > It would be good if kmap()/kunmap() can do this but looks like we can > not assume this? For example, sparc's flush_dcache_page()
Sorry, I meant kunmap_atomic().
Thanks
> doesn't do flush_dcache_page(). And bio_copy_data_iter() do > flush_dcache_page() after kunmap_atomic(). > > Thanks > > >> >> James >> >
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