Re: [PATCH] 9pfs: fix wrong I/O block size in Rgetattr

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 9/22/21 15:13, Christian Schoenebeck wrote:
> When client sent a 9p Tgetattr request then the wrong I/O block
> size value was returned by 9p server; instead of host file
> system's I/O block size it should rather return an I/O block
> size according to 9p session's 'msize' value, because the value
> returned to client should be an "optimum" block size for I/O
> (i.e. to maximize performance), it should not reflect the actual
> physical block size of the underlying storage media.
> 
> The I/O block size of a host filesystem is typically 4k, so the
> value returned was far too low for good 9p I/O performance.
> 
> This patch adds stat_to_iounit() with a similar approach as the
> existing get_iounit() function.
> 
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>   hw/9pfs/9p.c | 21 ++++++++++++++++++++-
>   1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index c857b31321..708b030474 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1262,6 +1262,25 @@ static int coroutine_fn stat_to_v9stat(V9fsPDU *pdu, V9fsPath *path,
>   #define P9_STATS_ALL           0x00003fffULL /* Mask for All fields above */
>   
>   
> +static int32_t stat_to_iounit(const V9fsPDU *pdu, const struct stat *stbuf)
> +{
> +    int32_t iounit = 0;
> +    V9fsState *s = pdu->s;
> +
> +    /*
> +     * iounit should be multiples of st_blksize (host filesystem block size)
> +     * as well as less than (client msize - P9_IOHDRSZ)
> +     */
> +    if (stbuf->st_blksize) {
> +        iounit = stbuf->st_blksize;
> +        iounit *= (s->msize - P9_IOHDRSZ) / stbuf->st_blksize;

Is that:

   iounit = QEMU_ALIGN_DOWN(s->msize - P9_IOHDRSZ, stbuf->st_blksize);

?

> +    }
> +    if (!iounit) {
> +        iounit = s->msize - P9_IOHDRSZ;
> +    }
> +    return iounit;
> +}
> +
>   static int stat_to_v9stat_dotl(V9fsPDU *pdu, const struct stat *stbuf,
>                                   V9fsStatDotl *v9lstat)
>   {
> @@ -1273,7 +1292,7 @@ static int stat_to_v9stat_dotl(V9fsPDU *pdu, const struct stat *stbuf,
>       v9lstat->st_gid = stbuf->st_gid;
>       v9lstat->st_rdev = stbuf->st_rdev;
>       v9lstat->st_size = stbuf->st_size;
> -    v9lstat->st_blksize = stbuf->st_blksize;
> +    v9lstat->st_blksize = stat_to_iounit(pdu, stbuf);
>       v9lstat->st_blocks = stbuf->st_blocks;
>       v9lstat->st_atime_sec = stbuf->st_atime;
>       v9lstat->st_atime_nsec = stbuf->st_atim.tv_nsec;
>