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

[Jason Wang]

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)
> > {
> >           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