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Date:   Tue, 12 Mar 2019 10:59:09 +0800
From:   Jason Wang <>
To:     David Miller <>,
Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through

On 2019/3/12 上午2:14, David Miller wrote:
> From: "Michael S. Tsirkin" <>
> 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);
         return 0;


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