Re: [Qemu-block] [PATCH] block: split large discard requests from block frontend

[Olaf Hering]

On Fri, May 06, Max Reitz wrote:

> On 01.04.2016 19:49, Olaf Hering wrote:
> > On Fri, Apr 01, Max Reitz wrote:
> > 
> >> In any case, do you have a test case where a guest was able to submit a
> >> request that led to the overflow error you described in the commit message?
> > 
> > mkfs -t ext4 /dev/sdb1 in a xen guest with qcow2 as backing device.
> > When I added discard support to libxl I worked with raw images, so I did
> > not notice this. Not sure why it happens to work in kvm guests. I assume
> > the frontend driver just works around the qemu bug by limiting its
> > request size.
> 
> Sorry for not having replied in so long.
> 
> I know next to nothing about Xen, but I'm very much inclined to think
> the Xen block driver (hw/block/xen_disk.c) is at fault here. The
> blkif_request_discard structure it uses for accepting discard requests
> apparently has a uint64_t nr_sectors field.

Thanks for the pointer.

Looking at current master, BLKIF_OP_DISCARD is indeed broken. The values
passed from the guest are u64 and get stashed into signed values. I will
add a loop to repeatedly call blk_aio_pdiscard with small chunks of
BDRV_REQUEST_MAX_SECTORS.

We quickly checked other users of blk_aio_pdiscard and it appears they
are not affected because they notify the guest abuilt the limits.

Olaf