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@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);
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
