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

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Message-ID: <f9e52313-0a06-22b6-140c-ded75eecde20@redhat.com>
Date:   Tue, 12 Mar 2019 15:51:53 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        David Miller <davem@...emloft.net>, mst@...hat.com
Cc:     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 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@...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);
> 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
>