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    SubjectRe: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
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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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