On Fri, Aug 04, 2017 at 07:08:54AM -0500, Eric Blake wrote: > On 08/04/2017 05:51 AM, Daniel P. Berrange wrote: > > Using 16KB bounce buffers creates a significant performance > > penalty for I/O to encrypted volumes on storage with high > > I/O latency (rotating rust & network drives), because it > > triggers lots of fairly small I/O operations. > > > > On tests with rotating rust, and cache=none|directsync, > > write speed increased from 2MiB/s to 32MiB/s, on a par > > with that achieved by the in-kernel luks driver. > > > > Signed-off-by: Daniel P. Berrange <berra...@redhat.com> > > --- > > block/crypto.c | 12 +++++------- > > 1 file changed, 5 insertions(+), 7 deletions(-) > > > > diff --git a/block/crypto.c b/block/crypto.c > > index 58ef6f2f52..207941db9a 100644 > > --- a/block/crypto.c > > +++ b/block/crypto.c > > @@ -379,7 +379,7 @@ static void block_crypto_close(BlockDriverState *bs) > > } > > > > > > -#define BLOCK_CRYPTO_MAX_SECTORS 32 > > +#define BLOCK_CRYPTO_MAX_SECTORS 2048 > > > > static coroutine_fn int > > block_crypto_co_readv(BlockDriverState *bs, int64_t sector_num, > > @@ -396,9 +396,8 @@ block_crypto_co_readv(BlockDriverState *bs, int64_t > > sector_num, > > > > qemu_iovec_init(&hd_qiov, qiov->niov); > > > > - /* Bounce buffer so we have a linear mem region for > > - * entire sector. XXX optimize so we avoid bounce > > - * buffer in case that qiov->niov == 1 > > + /* Bounce buffer because we're not permitted to touch > > + * contents of qiov - it points to guest memory. > > The comment updates are accurate (and in line with your other patch for > improving documentation of the callback semantics), but slightly > unrelated to the fix at hand. However, I have no problem keeping it in > the patch. > > (To make sure I understand the importance of the bounce buffer: On > reads, we can't store into the buffer until we have decrypted, so that > the guest can't transiently spy on the encrypted form; on writes, we > must read from the buffer at most once before encrypting, so that the > guest can't change the buffer under our feet while we are encrypting.)
For writes it is even more critical - if we encrypted in place, and the guest tried to serve a later read from its cache, it'd return cipher text instead of plain text. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|