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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
In-Reply-To: <20110610103305.GD18497@redhat.com>
References: <20110610103305.GD18497@redhat.com>
Date: Fri, 10 Jun 2011 17:36:13 +0530
Message-ID: <87y619evre.fsf@linux.vnet.ibm.com>
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Subject: Re: [Qemu-devel] QEMU 9pfs intentionally returning short reads ?
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To: "Daniel P. Berrange" <berrange@redhat.com>, qemu-devel@nongnu.org

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

-aneesh