On 2019/5/8 下午12:12, Michael S. Tsirkin wrote:
On Mon, May 06, 2019 at 10:23:29PM -0400, Jason Wang wrote:
Vhost log dirty pages directly to a userspace bitmap through GUP and
kmap_atomic() since kernel doesn't have a set_bit_to_user()
helper. This will cause issues for the arch that has virtually tagged
caches. The way to fix is to keep using userspace virtual address.
Fortunately, futex has a cmpxchg to userspace memory helper
futex_atomic_cmpxchg_inatomic(). So switch to use it to exchange the
userspace bitmap with zero, set the bit and then write it back through
put_user().
Note: there're archs (few non popular ones) that don't implement
futex helper, we can't log dirty pages. We can fix them on top or
simply disable LOG_ALL features of vhost.
Or implement futex_atomic_cmpxchg using kmap if they don't have
virtually tagged caches.
Yes, this might work.
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
drivers/vhost/vhost.c | 27 +++++++++++++++------------
1 file changed, 15 insertions(+), 12 deletions(-)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 351af88..9c94c41 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -31,6 +31,7 @@
#include <linux/sched/signal.h>
#include <linux/interval_tree_generic.h>
#include <linux/nospec.h>
+#include <asm/futex.h>
#include "vhost.h"
@@ -1692,25 +1693,27 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
}
EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
-/* TODO: This is really inefficient. We need something like get_user()
- * (instruction directly accesses the data, with an exception table entry
- * returning -EFAULT). See Documentation/x86/exception-tables.txt.
- */
-static int set_bit_to_user(int nr, void __user *addr)
+static int set_bit_to_user(int nr, u32 __user *addr)
{
unsigned long log = (unsigned long)addr;
struct page *page;
- void *base;
- int bit = nr + (log % PAGE_SIZE) * 8;
+ u32 old_log;
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);
+
+ r = futex_atomic_cmpxchg_inatomic(&old_log, addr, 0, 0);
+ if (r < 0)
+ return r;
So I think this is a great idea!
However one issue here is that futex_atomic_cmpxchg_inatomic will fail if the
page is swapped out. I suspect we need a variant that blocks the thread
instead.
I guess not since the patch still try to pin the page before.
Thanks
+
+ old_log |= 1 << nr;
+ r = put_user(old_log, addr);
+ if (r < 0)
+ return r;
+
set_page_dirty_lock(page);
put_page(page);
return 0;
@@ -1727,8 +1730,8 @@ static int log_write(void __user *log_base,
write_length += write_address % VHOST_PAGE_SIZE;
for (;;) {
u64 base = (u64)(unsigned long)log_base;
- u64 log = base + write_page / 8;
- int bit = write_page % 8;
+ u64 log = base + write_page / 32;
+ int bit = write_page % 32;
if ((u64)(unsigned long)log != log)
return -EFAULT;
r = set_bit_to_user(bit, (void __user *)(unsigned long)log);
--
1.8.3.1
_______________________________________________
Virtualization mailing list
Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
