Re: [PATCH] xen-blkfront: don't make discard-alignment mandatory

On Tue, Jan 19, 2021 at 11:11:26AM +0100, Jürgen Groß wrote:

On 19.01.21 11:06, Roger Pau Monné wrote:

quoted

On Tue, Jan 19, 2021 at 08:43:01AM +0100, Jürgen Groß wrote:

quoted

On 18.01.21 16:15, Roger Pau Monne wrote:

quoted

Don't require the discard-alignment xenstore node to be present in
order to correctly setup the feature. This can happen with versions of
QEMU that only write the discard-granularity but not the
discard-alignment node.

Assume discard-alignment is 0 if not present. While there also fix the
logic to not enable the discard feature if discard-granularity is not
present.

Reported-by: Arthur Borsboom <redacted>
Signed-off-by: Roger Pau Monné <redacted>
---
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Konrad Rzeszutek Wilk <redacted>
Cc: "Roger Pau Monné" <redacted>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: xen-devel@lists.xenproject.org
Cc: linux-block@vger.kernel.org
Cc: Arthur Borsboom <redacted>
---
   drivers/block/xen-blkfront.c | 25 +++++++++++++------------
   1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 5265975b3fba..5a93f7cc2939 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c

@@ -2179,22 +2179,23 @@ static void blkfront_closing(struct blkfront_info *info)
   static void blkfront_setup_discard(struct blkfront_info *info)
   {
-	int err;
-	unsigned int discard_granularity;
-	unsigned int discard_alignment;
+	unsigned int discard_granularity = 0;
+	unsigned int discard_alignment = 0;
+	unsigned int discard_secure = 0;
-	info->feature_discard = 1;
-	err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+	xenbus_gather(XBT_NIL, info->xbdev->otherend,
   		"discard-granularity", "%u", &discard_granularity,
   		"discard-alignment", "%u", &discard_alignment,
+		"discard-secure", "%u", &discard_secure,
   		NULL);

This would mean that "discard-secure" will be evaluated only if the
other two items are set in Xenstore. From blkif.h I can't see this is
required, and your patch is modifying today's behavior in this regard.

You might want to have three xenbus_read_unsigned() calls instead.

You are right, discard-secure should be fetched regardless of whether
discard-alignment exists.

I can fetch discard-granularity and discard-alignment using
xenbus_gather and keep discard-secure using xenbus_read_unsigned. Let
me send a new version.

I'm still not convinced this is correct. blkif.h doesn't mention that
discard-alignment will be valid only with discard-granularity being
present.

No, in fact I think I need to rework this a little further. Just
having feature-discard = 1 should enable the discard functionality, by
setting discard-granularity = physical block size and
discard-alignment = 0.

Roger.