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@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 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
