Re: [PATCH 2/2] kvm tools: Add virtio-9p

Venkateswararao Jujjuri
2011-05-26T14:28:56Z

On 05/18/2011 02:05 AM, Sasha Levin wrote:
> On Tue, 2011-05-17 at 20:18 -0500, Eric Van Hensbergen wrote:
>> On Tue, May 17, 2011 at 3:27 PM, Sasha Levin<levinsasha928@gmail.com> wrote:
>>> On Tue, 2011-05-17 at 22:08 +0300, Sasha Levin wrote:
>>>> 'kvm_9p' isn't created as a device under /dev, it's just a name used
>>>> internally by 9pnet_virtio (and located under sysfs).
>>>>
>>>> I couldn't figure out which params the kernel would expect to boot using
>>>> 9p over virtio (theres no device name to begin with).
>>>>
>>>> I've also couldn't find anything that suggested it's possible to boot
>>>> using virtio-9p as rootfs.
>>> Ignore that.
>>>
>>> Naming the virtio transport "/dev/root" and passing proper params to the
>>> kernel makes it work:
>>>
>>> [ 1.844983] VFS: Mounted root (9p filesystem) on device 0:11.
>>>
>>> I'll make some changes to the virtio-9p patch to make it easier for the
>>> user to do that.

Any progress on this? May I get more detailed instructions on how
you did this trick? Basically booting on 9P/VirtIO.

Thanks,
JV

>> This is really sweet. Thanks for beating me to the punch of porting
>> the 9p support to kvm tools.
> Clear RFC and good source code to refer to within 9p modules made this
> easy (and fun) :)
>
>>> - Multiple virtio-9p devices.
>> This should be pretty straightforward.
> Yes, Most of the work here is within the kvm tool.
>
>>> - Ugly hack in virtio_p9_stat() (See desc in code).
>>> /*
>>> + * HACK: For some reason the p9 virtio transport reads a u16 and discards
>>> + * it before reading the p9_rstat struct. I couldn't find a logical reason for
>>> + * that, so we just add an extra u16 before the struct.
>>> + */
>> This is part of the protocol spec (from
>> http://ericvh.github.com/9p-rfc/rfc9p2000.html#anchor32):
>> "To make the contents of a directory, such as returned by read(5),
>> easy to parse, each directory entry begins with a size field. For
>> consistency, the entries in Twstat and Rstat messages also contain
>> their size, which means the size appears twice. For example, the Rstat
>> message is formatted as ``(4+1+2+2+n)[4] Rstat tag[2] n[2] (n-2)[2]
>> type[2] dev[4]...,'' where n is the value returned by convD2M."
>> It's appropriate to duplicate the size. I think the Linux client
>> ignores it, but others implementations may complain.
> Thanks for the explanation!
> Yes, Linux implementation just throws it away - which was what confused
> me initially.
> Why not add a u16 to the beginning of 'struct p9_rstat'?
>
>>> - Update atime/mtime in p9_wstat, not really needed.
>> The underlying storage may handle this for you, I think 9p avoids
>> updating atime by default, at least in caching scenarios -- too much
>> unnecessary protocol traffic.
> My assumption was that the storage I read/write to will take care of it
> for me, and unless it bothers anyone in the future I'll assume it's
> doing a good job at it.
>
>>> - Pass usernames in p9_stat, not really needed and not really sure how p9 expects to handle them.
>> The username, group name issue is one of the principle reasons behind
>> the extended protocol operations (.u and .L) -- of course, if there
>> was a Plan 9 or Inferno guest they would be quite happy with the
>> usernames, but Linux (and other UNIX variants) will want the ids. To
>> really keep things simple we could add a client option that would let
>> you pass the various ids as strings. Although no doubt folks will
>> want the other extensions (symlinks, links, device nodes, etc.) before
>> long. When we built the qemu server for .L, the team tried to keep
>> everything in a library, but there is some entanglement with the qemu
>> APIs -- it'd be nice if we could reuse that code here, maybe we need
>> an abstract glue layer so that the core code can be used by both the
>> kvm tool and qemu. I'm copy the lead of that team on this message
>> just so he's aware how far you've come.
> I'd prefer using a tested lib which also implements .L over what we have
> now, assuming it's not tangled into qemu too hard.
>
>
>> -eric
>