Using 16KB bounce buffers creates a significant performance penalty for I/O to encrypted volumes on storage which 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. With other cache modes the in-kernel driver is still notably faster because it is able to report completion of the I/O request before any encryption is done, while the in-QEMU driver must encrypt the data before completion. Reviewed-by: Eric Blake <ebl...@redhat.com> Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com> 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..cc8afe0e0d 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 don't wish to expose cipher text + * in qiov which points to guest memory. */ cipher_data = qemu_try_blockalign(bs->file->bs, MIN(BLOCK_CRYPTO_MAX_SECTORS * 512, @@ -464,9 +463,8 @@ block_crypto_co_writev(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, -- 2.13.5