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

[Daniel P. Berrange]

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" <address@hidden> 
> 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

Regards,
Daniel
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