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

[Jason Wang]

On 2019/3/12 äå3:51, Jason Wang wrote:
> On 2019/3/12 äå1:14, James Bottomley wrote:
> > On Tue, 2019-03-12 at 10:59 +0800, Jason Wang wrote:
> > > On 2019/3/12 äå2:14, David Miller wrote:
> > > > From: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> > > > 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 äå10:12, 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)
> > > {
> > > ÂÂÂÂÂÂÂÂÂ unsigned long log = (unsigned long)addr;
> > > ÂÂÂÂÂÂÂÂÂ struct page *page;
> > > ÂÂÂÂÂÂÂÂÂ void *base;
> > > ÂÂÂÂÂÂÂÂÂ int bit = nr + (log % PAGE_SIZE) * 8;
> > > ÂÂÂÂÂÂÂÂÂ int r;
> > >
> > > ÂÂÂÂÂÂÂÂÂ r = get_user_pages_fast(log, 1, 1, &page);
> > > ÂÂÂÂÂÂÂÂÂ if (r < 0)
> > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return r;
> > > ÂÂÂÂÂÂÂÂÂ BUG_ON(r != 1);
> > > ÂÂÂÂÂÂÂÂÂ base = kmap_atomic(page);
> > > ÂÂÂÂÂÂÂÂÂ set_bit(bit, base);
> > > ÂÂÂÂÂÂÂÂÂ 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() 


Sorry, I meant kunmap_atomic().

Thanks


> doesn't do flush_dcache_page(). And bio_copy_data_iter() do 
> flush_dcache_page() after kunmap_atomic().
>
> Thanks
>
>
> > James