Re: [Xen-devel] [PATCH 5/8] libxl: Share logic for finding path between qemuu and pygrub

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Mar 17, 2016 at 6:22 PM, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> George Dunlap writes ("[PATCH 5/8] libxl: Share logic for finding path 
> between qemuu and pygrub"):
>> From: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>
> Thanks.  There are a few format inconsistencies (long lines, spaces)
> which I won't enumerate.
>
>> * In the case of a block script, or a non-dom0 backend, qemuu will now
>> print a warning and skip the disk, rather than adding bogus
>> parameters that will cause qemuu to error out.
>
> I think the consequences here ought to be better spelled out.  AFAICT
> the implication is that, in this case, the disk will be available to
> the guest as a PV disk (because libxl will select a backend other than
> qdisk) but not via the emulated IDE ?
>
> IWBNI the resulting functional restriction were documented, but I
> won't insist on that given that you're improving matters.
>
>> @@ -3023,6 +3024,16 @@ static char * 
>> libxl__device_disk_find_local_path(libxl__gc *gc,
>>          goto out;
>>      }
>>
>> +    /*
>> +     * If we're being called for a qemu path, we can pass the target
>> +     * string directly as well
>> +     */
>> +    if (qdisk_direct && disk->backend == LIBXL_DISK_BACKEND_QDISK ) {
>> +        path = libxl__strdup(gc, disk->pdev_path);
>> +        LOG(DEBUG, "Directly accessing local QDISK target %s", path);
>> +        goto out;
>
> Is this really true ?  What if the format is qcow2 ?  I think that
> might result in feeding pygrub the qcow2 file rather than the virtual
> block image it contains.

That's what the "qdisk_direct" argument is for -- it tells
libxl__device_disk_find_local_path, "I can interpret QDISK backends".
Pygrub will call this function with "qdisk_direct" set to "false",
since it can't interpret QDISK backends; this will result in it doing
a local attach instead.

> Overall I think this patch is otherwise probably good but it it's a
> bit hard to see the wood for the trees.  If you felt like factoring
> out some of the refactoring and variable renaming, from the functional
> change, that would be very nice.

Yes, there are a lot of things going on, aren't there.  I think I
remember trying to make it simpler when I first wrote it last year,
but not finding any really sensible in-between points; but I'll give
it another look.

 -George

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