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Re: [Qemu-devel] [PATCH for 2.10] block: use 1 MB bounce buffers for cry
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH for 2.10] block: use 1 MB bounce buffers for crypto instead of 16KB |
Date: |
Fri, 4 Aug 2017 13:48:01 +0100 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
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 <address@hidden>
> ---
> 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.
Reviewed-by: Stefan Hajnoczi <address@hidden>
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