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 15:51:53 +0800 |
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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() doesn't do flush_dcache_page(). And bio_copy_data_iter() do flush_dcache_page() after kunmap_atomic().
Thanks
> > James >
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