Re: [Qemu-devel] QEMU 9pfs intentionally returning short reads ?

[Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>]

On 06/10/2011 05:20 AM, Daniel P. Berrange wrote:
> On Fri, Jun 10, 2011 at 05:36:13PM +0530, Aneesh Kumar K.V wrote:
>> On Fri, 10 Jun 2011 11:33:05 +0100, "Daniel P. Berrange"<berrange@redhat.com>  wrote:
>>> I've been doing some work trying to run QEMU guests with a root filesystem
>>> exported from the host using virtio 9pfs. One of the issues that I have
>>> discovered is that the 9p FS running on QEMU appears to cap all reads at
>>> 4096 bytes[1]. Any larger read will return only partial data for plain
>>> files.
>>>
>> But we should loop in kernel, requesting for multiple 9p request.
>>
>> kernel does
>>
>> 	size = fid->iounit ? fid->iounit : fid->clnt->msize - P9_IOHDRSZ;
>> 	if (count>  size)
>> 		ret = v9fs_file_readn(filp, NULL, udata, count, *offset);
>> 	else
>> 		ret = p9_client_read(fid, NULL, udata, *offset, count);
>>
>> and v9fs_file_readn() does..
>>
>> 	do {
>> 		n = p9_client_read(fid, data, udata, offset, count);
>> 		if (n<= 0)
>> 			break;
>>
>> 		if (data)
>> 			data += n;
>> 		if (udata)
>> 			udata += n;
>>
>> 		offset += n;
>> 		count -= n;
>> 		total += n;
>> 	} while (count>  0&&  n == size);
>>
>>
>> I also did an strace of simple test and i see
>>
>> open("test", O_RDONLY)                  = 3
>> read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 8192) = 8192
> In my test I did
>
> #  strace -e trace=read,open perl -e 'open FOO, "/usr/share/X11/XKeysymDB"; sysread FOO, $foo, 8192'
> open("/usr/share/X11/XKeysymDB", O_RDONLY) = 3
> read(3, "! Copyright 1993 Massachusetts I"..., 8192) = 4096
>
> Perhaps there is a guest kernel driver difference ? I'm using
>
>    2.6.35.13-91.fc14.x86_64
The default msize is 8k; Because of iounit the net layer need to send 
the IO in the chunks of 4k
8k - header size faults into 4k; But never the less the fs layer should 
loop.

There was an issue with read; and that bug is fixed by
commit 97e8442b0971ea6be9a495b3d03402985cfe5d6a
Author: M. Mohan Kumar <mohan@in.ibm.com>
Date:   Fri Jun 4 11:59:07 2010 +0000

     9p: Make use of iounit for read/write

     Change the v9fs_file_readn function to limit the maximum transfer size
     based on the iounit or msize.

     Also remove the redundant check for limiting the transfer size in
     v9fs_file_write. This check is done by p9_client_write.

     Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
     Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>



Thanks,
JV
> Regards,
> Daniel