Re: [Qemu-devel] [PATCH for-2.5 1/4] block: add BlockLimits.max_iov field

[Peter Lieven <pl@kamp.de>]

Am 08.07.2015 um 17:30 schrieb Stefan Hajnoczi:
> The maximum number of struct iovec elements depends on the
> BlockDriverState.  The raw-posix protocol has a maximum of IOV_MAX but
> others could have different values.
>
> Instead of assuming raw-posix and hardcoding IOV_MAX in several places,
> put the limit into BlockLimits.
>
> Cc: Peter Lieven <pl@kamp.de>
> Suggested-by: Kevin Wolf <kwolf@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> Peter Lieven: I think the SCSI LUN level does not have a maximum
> scatter-gather segments constraint.  That is probably only at the HBA
> level.  CCed you anyway in case you think block/iscsi.c should set the
> max_iov field.

libiscsi will send the iovec array straight to readv and writev to
read/write from the TCP socket. So we need IOV_MAX here as well.

>
> Kevin: The default is now INT_MAX.  This means non-raw-posix users will
> now be able to merge more requests than before.  They were limited to
> IOV_MAX previously.  This could expose limits in other BlockDrivers
> which we weren't aware of...

Why rise the default to INT_MAX and not leave it at IOV_MAX?
Is there any case where we except that much iovectors coming in?

Peter