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

[Aneesh Kumar K.V]

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

-aneesh