Re: [Xen-devel] [PATCH] blktap2: Invalid logic detecting unaligned buffers in vhd_write_block

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Wed, Apr 6, 2016 at 5:51 PM, Ross Philipson <ross.philipson@xxxxxxxxx> wrote:
> On 03/29/2016 07:26 AM, George Dunlap wrote:
>> On Fri, Mar 25, 2016 at 4:34 PM, Ross Philipson
>> <ross.philipson@xxxxxxxxx> wrote:
>>> On 03/25/2016 12:11 PM, Wei Liu wrote:
>>>> On Thu, Mar 17, 2016 at 06:10:59PM -0400, Ross Philipson wrote:
>>>>> It seems the logic is meant to detect sector unaligned buffers for block
>>>>> writes. The NOTing of the logic instead masks off any unaligned bits and
>>>>> also would cause the function to always fail. It seems the function is not
>>>>> used in any of the tools so that is probably why the problem is not seen.
>>>>> In the vhd_read_block function it is correct.
>>>>>
>>>>> Signed-off-by: Ross Philipson <ross.philipson@xxxxxxxxxxxx>
>>>>
>>>> This seems to fall under tools umbrella. I've look at blktap2 code,
>>>> the reasoning is convincing to me so:
>>>>
>>>>   Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>>
>>>> But I've never used blktap2 and we don't normally test it so I would
>>>> also wait a bit longer to see if anybody objects to this change.
>>>>
>>>> OOI if no code was using this, how did you discover this problem?
>>>
>>> We have an old fork of blktap2 from way back when. I was working to extract 
>>> our
>>> changes and turn them into patches so we could rebase on upstream blktap2.
>>> Someone long ago found that - I have no idea how but it was a valid fix so I
>>> upstreamed it.
>>>
>>> There are a number of other ones that were found that are still in upstream
>>> blktap2 - I plan to send them too.
>>
>> Ross,
>>
>> Instead of cross-porting fixes from XenServer's blktap3 to the
>> bitrotting blktap2, would you consider taking up my work on replacing
>> blktap2 with building blktap3 as an external project?
>
> George, a few questions below to get started...
>
>>
>> The basic shape of things that need to happen for that are:
>>
>> 1. Allow qemu to provide emulated devices for disks which use hotplug
>> scripts (just checked in last week)
>>
>> 2. Do the tweaks necessary to allow blktap3 to be built as an
>> independent project (outside the XenServer build system)
>
> I am not sure where the latest and greatest blktap3 work can be found. I have
> found some old repos that have activity up until the work stopped on that
> project in 2013. Can you give me some pointers on this?
>
> Also it seems the overall idea is to not have blktap3 (or any blktap) live in
> the Xen tree but rather be external. Where would blktap3 live?
>
>>
>> 3. Switch libxl to use the already-in-tree block-tap script (which
>> calls tapctl) rather than linking against the blktap library.
>
> I believe I found this work:
>
> http://lists.xenproject.org/archives/html/xen-devel/2015-07/msg00805.html
>
> It looks like 4 of the set made it in and patches 3 & 5 still need work? I can
> re-base them.

Hey Ross,

Sorry for dropping this on the floor -- are you still interested in
this work?  If so I'll take some time to summarize where I got to. :-)

 -George

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