On Fri, Aug 04, 2017 at 01:48:01PM +0100, Stefan Hajnoczi wrote: > On Fri, Aug 04, 2017 at 11:51:36AM +0100, 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. > > */ > > cipher_data = > > qemu_try_blockalign(bs->file->bs, MIN(BLOCK_CRYPTO_MAX_SECTORS * > > 512, > > In the *read* case you can modify the data buffers in-place. But the > guest might see intermediate states in its buffers - not sure whether > this could pose a security problem.
Whether its a risk or not depends on the choice of crypto parameters, as exposing ciphertext to the guest might make watermarking attacks easier to perform. Probably not a problem in practice, but I prefer to err on the side of caution since I can't be sure it is safe. > Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com> 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 :|
