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    SubjectRe: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
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    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);
            set_page_dirty_lock(page);
            put_page(page);
            return 0;
    }

    Thanks

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