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

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:


It looks like 4 of the set made it in and patches 3 & 5 still need work? I can
re-base them.

> 4. Add a blktap target to raisin.
> #1 is done and was just checked in last week.  #2 shouldn't be a
> terribly large amount of work.  For #3, I've posted patches already,
> and I can probably do a quick rebase for you to pick up if you wanted.
> #4 shouldn't be too hard once you have an out-of-tree build working; I
> can help with that as well.
> If we then add tests for upstream blktap to osstest, then we'll catch
> any breakages, and automatically get updates; and we can delete the
> bitrotting blktap2 out of the tree.
> What do you think?
>  -George

Ross Philipson

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