Re: [Xen-devel] [PATCH 2/2] xen: dont try setting max grants multiple times

[Juergen Gross <jgross@xxxxxxxx>]

On 20/09/17 17:00, Anthony PERARD wrote:
> On Tue, Sep 19, 2017 at 01:50:55PM +0200, Juergen Gross wrote:
>> Trying to call xengnttab_set_max_grants() with the same file handle
>> might fail on some kernels, as this operation is allowed only once.
>>
>> This is a problem for the qdisk backend as blk_connect() can be
>> called multiple times for a domain, e.g. in case grub-xen is being
>> used to boot it.
>>
>> So instead of letting the generic backend code open the gnttab device
>> do it in blk_connect() and close it again in blk_disconnect.
>>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>> ---
>>  hw/block/xen_disk.c | 12 +++++++++++-
>>  1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
>> index 6632746250..7cff8863cb 100644
>> --- a/hw/block/xen_disk.c
>> +++ b/hw/block/xen_disk.c
>> @@ -1220,6 +1220,12 @@ static int blk_connect(struct XenDevice *xendev)
>>      /* Add on the number needed for the ring pages */
>>      max_grants += blkdev->nr_ring_ref;
>>  
>> +    blkdev->xendev.gnttabdev = xengnttab_open(NULL, 0);
>> +    if (blkdev->xendev.gnttabdev == NULL) {
>> +        xen_pv_printf(xendev, 0, "xengnttab_open failed: %s\n",
>> +                      strerror(errno));
>> +        return -1;
>> +    }
>>      if (xengnttab_set_max_grants(blkdev->xendev.gnttabdev, max_grants)) {
>>          xen_pv_printf(xendev, 0, "xengnttab_set_max_grants failed: %s\n",
>>                        strerror(errno));
>> @@ -1327,6 +1333,11 @@ static void blk_disconnect(struct XenDevice *xendev)
>>          }
>>          blkdev->feature_persistent = false;
>>      }
>> +
>> +    if (blkdev->xendev.gnttabdev) {
>> +        xengnttab_close(blkdev->xendev.gnttabdev);
>> +        blkdev->xendev.gnttabdev = NULL;
>> +    }
> 
> I think blk_disconnect needs to be called from blk_free in case where
> the gnttabdev is not closed (like it is done when blk or the sring are
> not cleared, in blk_free).

Right. Just calling it always from blk_free() should do the job.

BTW: in practice this wouldn't be necessary as xen_pv_del_xendev()
already does the close, but this is just good luck. Better closing it
via blk_free() in case xen_pv_del_xendev() adds a test for
DEVOPS_FLAG_NEED_GNTDEV.


Thanks,

Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel