Re: [PATCH v2 4/4] xen-blkback: support dynamic unbind/bind

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Dec 10, 2019 at 11:33:47AM +0000, Paul Durrant wrote:
> By simply re-attaching to shared rings during connect_ring() rather than
> assuming they are freshly allocated (i.e assuming the counters are zero)
> it is possible for vbd instances to be unbound and re-bound from and to
> (respectively) a running guest.
> 
> This has been tested by running:
> 
> while true;
>   do fio --name=randwrite --ioengine=libaio --iodepth=16 \
>   --rw=randwrite --bs=4k --direct=1 --size=1G --verify=crc32;
>   done
> 
> in a PV guest whilst running:
> 
> while true;
>   do echo vbd-$DOMID-$VBD >unbind;
>   echo unbound;
>   sleep 5;

Is there anyway to know when the unbind has finished? AFAICT
xen_blkif_disconnect will return EBUSY if there are in flight
requests, and the disconnect won't be completed until those requests
are finished.

>   echo vbd-$DOMID-$VBD >bind;
>   echo bound;
>   sleep 3;
>   done
> 
> in dom0 from /sys/bus/xen-backend/drivers/vbd to continuously unbind and
> re-bind its system disk image.
> 
> This is a highly useful feature for a backend module as it allows it to be
> unloaded and re-loaded (i.e. updated) without requiring domUs to be halted.
> This was also tested by running:
> 
> while true;
>   do echo vbd-$DOMID-$VBD >unbind;
>   echo unbound;
>   sleep 5;
>   rmmod xen-blkback;
>   echo unloaded;
>   sleep 1;
>   modprobe xen-blkback;
>   echo bound;
>   cd $(pwd);
>   sleep 3;
>   done
> 
> in dom0 whilst running the same loop as above in the (single) PV guest.
> 
> Some (less stressful) testing has also been done using a Windows HVM guest
> with the latest 9.0 PV drivers installed.
> 
> Signed-off-by: Paul Durrant <pdurrant@amazon.com>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> 
> v2:
>  - Apply a sanity check to the value of rsp_prod and fail the re-attach
>    if it is implausible
>  - Set allow_rebind to prevent ring from being closed on unbind
>  - Update test workload from dd to fio (with verification)
> ---
>  drivers/block/xen-blkback/xenbus.c | 59 +++++++++++++++++++++---------
>  1 file changed, 41 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index e8c5c54e1d26..13d09630b237 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -181,6 +181,8 @@ static int xen_blkif_map(struct xen_blkif_ring *ring, grant_ref_t *gref,
>  {
>  	int err;
>  	struct xen_blkif *blkif = ring->blkif;
> +	struct blkif_common_sring *sring_common;
> +	RING_IDX rsp_prod, req_prod;
>  
>  	/* Already connected through? */
>  	if (ring->irq)
> @@ -191,46 +193,66 @@ static int xen_blkif_map(struct xen_blkif_ring *ring, grant_ref_t *gref,
>  	if (err < 0)
>  		return err;
>  
> +	sring_common = (struct blkif_common_sring *)ring->blk_ring;
> +	rsp_prod = READ_ONCE(sring_common->rsp_prod);
> +	req_prod = READ_ONCE(sring_common->req_prod);
> +
>  	switch (blkif->blk_protocol) {
>  	case BLKIF_PROTOCOL_NATIVE:
>  	{
> -		struct blkif_sring *sring;
> -		sring = (struct blkif_sring *)ring->blk_ring;
> -		BACK_RING_INIT(&ring->blk_rings.native, sring,
> -			       XEN_PAGE_SIZE * nr_grefs);
> +		struct blkif_sring *sring_native =
> +			(struct blkif_sring *)ring->blk_ring;

I think you can constify both sring_native and sring_common (and the
other instances below).

> +		unsigned int size = __RING_SIZE(sring_native,
> +						XEN_PAGE_SIZE * nr_grefs);
> +
> +		BACK_RING_ATTACH(&ring->blk_rings.native, sring_native,
> +				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
> +		err = (req_prod - rsp_prod > size) ? -EIO : 0;
>  		break;
>  	}
>  	case BLKIF_PROTOCOL_X86_32:
>  	{
> -		struct blkif_x86_32_sring *sring_x86_32;
> -		sring_x86_32 = (struct blkif_x86_32_sring *)ring->blk_ring;
> -		BACK_RING_INIT(&ring->blk_rings.x86_32, sring_x86_32,
> -			       XEN_PAGE_SIZE * nr_grefs);
> +		struct blkif_x86_32_sring *sring_x86_32 =
> +			(struct blkif_x86_32_sring *)ring->blk_ring;
> +		unsigned int size = __RING_SIZE(sring_x86_32,
> +						XEN_PAGE_SIZE * nr_grefs);
> +
> +		BACK_RING_ATTACH(&ring->blk_rings.x86_32, sring_x86_32,
> +				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
> +		err = (req_prod - rsp_prod > size) ? -EIO : 0;
>  		break;
>  	}
>  	case BLKIF_PROTOCOL_X86_64:
>  	{
> -		struct blkif_x86_64_sring *sring_x86_64;
> -		sring_x86_64 = (struct blkif_x86_64_sring *)ring->blk_ring;
> -		BACK_RING_INIT(&ring->blk_rings.x86_64, sring_x86_64,
> -			       XEN_PAGE_SIZE * nr_grefs);
> +		struct blkif_x86_64_sring *sring_x86_64 =
> +			(struct blkif_x86_64_sring *)ring->blk_ring;
> +		unsigned int size = __RING_SIZE(sring_x86_64,
> +						XEN_PAGE_SIZE * nr_grefs);
> +
> +		BACK_RING_ATTACH(&ring->blk_rings.x86_64, sring_x86_64,
> +				 rsp_prod, XEN_PAGE_SIZE * nr_grefs);
> +		err = (req_prod - rsp_prod > size) ? -EIO : 0;

This is repeated for all ring types, might be worth to pull it out of
the switch...

>  		break;
>  	}
>  	default:
>  		BUG();
>  	}
> +	if (err < 0)
> +		goto fail;

...and placed here instead?

Thanks, Roger.