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Subject: Re: [PATCH v3 4/4] xen-blkback: support dynamic unbind/bind
To:     Paul Durrant <pdurrant@amazon.com>, xen-devel@lists.xenproject.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
        Jens Axboe <axboe@kernel.dk>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>
References: <20191211152956.5168-1-pdurrant@amazon.com>
 <20191211152956.5168-5-pdurrant@amazon.com>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <d3c61bd0-c29f-bb73-f637-158c7541a5f3@suse.com>
Date:   Thu, 12 Dec 2019 07:07:06 +0100
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On 11.12.19 16:29, 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;
>    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>

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen