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

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Date:   Tue, 19 Jan 2021 11:11:26 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
Cc:     linux-kernel@...r.kernel.org,
        Arthur Borsboom <arthurborsboom@...il.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Jens Axboe <axboe@...nel.dk>, xen-devel@...ts.xenproject.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCH] xen-blkfront: don't make discard-alignment mandatory

On 19.01.21 11:06, Roger Pau Monné wrote:
> On Tue, Jan 19, 2021 at 08:43:01AM +0100, Jürgen Groß wrote:
>> On 18.01.21 16:15, Roger Pau Monne wrote:
>>> 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 <arthurborsboom@...il.com>
>>> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
>>> ---
>>> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> Cc: Juergen Gross <jgross@...e.com>
>>> Cc: Stefano Stabellini <sstabellini@...nel.org>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>>> Cc: "Roger Pau Monné" <roger.pau@...rix.com>
>>> Cc: Jens Axboe <axboe@...nel.dk>
>>> Cc: xen-devel@...ts.xenproject.org
>>> Cc: linux-block@...r.kernel.org
>>> Cc: Arthur Borsboom <arthurborsboom@...il.com>
>>> ---
>>>    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.


Juergen

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