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Date:   Tue, 12 Mar 2019 15:17:00 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     David Miller <davem@...emloft.net>, hch@...radead.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterx@...hat.com, linux-mm@...ck.org, aarcange@...hat.com,
        linux-arm-kernel@...ts.infradead.org, linux-parisc@...r.kernel.org
Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through
 vmap()


On 2019/3/12 上午11:52, Michael S. Tsirkin wrote:
> On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote:
>> On 2019/3/12 上午2:14, David Miller wrote:
>>> From: "Michael S. Tsirkin" <mst@...hat.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
> I think you are right. The correct fix though is to re-implement
> it using asm and handling pagefault, not gup.


I agree but it needs to introduce new helpers in asm  for all archs 
which is not trivial. At least for -stable, we need the flush?


> Three atomic ops per bit is way to expensive.


Yes.

Thanks

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