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Re: [Qemu-devel] [PATCH 2/4] block: immediately cancel oversized read/wr


From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH 2/4] block: immediately cancel oversized read/write requests
Date: Mon, 08 Sep 2014 17:16:25 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.0

On 08.09.2014 17:13, ronnie sahlberg wrote:
On Mon, Sep 8, 2014 at 7:42 AM, Paolo Bonzini <address@hidden> wrote:
Il 08/09/2014 16:35, Peter Lieven ha scritto:
messages. :)
So you would not throw an error msg here?
No, though a trace could be useful.
Is there a howto somewhere how to implement that?
Try commit 4ac4458076e1aaf3b01a6361154117df20e22215.

Whats your opinion changed the max_xfer_len to 0xffff regardsless
of use_16_for_rw in iSCSI?
If you implemented request splitting in the block layer, it would be
okay to force max_xfer_len to 0xffff.
I think it should be OK if that is a different series. This only
affects the iSCSI transport since it is the only transport so far to
record or report a max transfer length.
If a guest generates these big requests(i.e. not multiwrite) we
already fail them due to the READ10 limitations. We would just fail
the request at a different layer in the stack with these patches.


What I would like to see would also be to report these limitations to
the guest itself to prevent it from generating too large I/Os.
I am willing to do that part once the initial max_xfer_len ones are finished.

Yes, I also think this approach is straightforward. We should avoid
big requests at the beginning and not fix them at the end.
I had a look it shouldn't be too hard. There are default values for
virtio-scsi HD inquiry emulation which are set if no cmdline values
are specified. It should be possible to feed them from bs->bl if set.

Peter



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