Re: [Qemu-devel] [Nbd] [PATCH 3/1] doc: Propose Structured Replies extension

[Eric Blake]

On 03/29/2016 12:03 PM, Wouter Verhelst wrote:
> On Tue, Mar 29, 2016 at 11:45:45AM -0600, Eric Blake wrote:
>> On 03/29/2016 11:34 AM, Alex Bligh wrote:
>>> I would agree. I think if it supports the structured reply semantics,
>>> it should also support 'DF'. So if you know the server supports
>>> structured replies, you know you can set DF on them without any
>>> further requirements.
>>
>> Supporting DF merely transfers the burden of collection between server
>> and client.  I suspect that there are cases where the server does NOT
>> want to support DF (because it would require the server to allocate
>> memory to collect the data before sending a single structured read
>> reply),
> 
> There are other ways to handle that; e.g., the server could have a
> "request too large for non-fragmented read" error message. The spec
> should give a minimum size that the server MUST support (which should be
> reasonably large), and should state that a server MAY reply to any
> request with DF set for a block larger than that minimum, with that
> error.

How does 64k sound?

> 
> Otherwise the client could conceivably send out heaps of requests for
> (UINT32_MAX - 8) bytes with DF set and basically DoS the server.

Point taken.

> 
>> just as you have stated that there are cases where the client
>> has an additional burden if DF is not supported.  So for v2, I'm going
>> to explicitly document a DF export flag, and recommend (but not require)
>> that the server support it.
> 
> I'd prefer not to see that.

Okay, good thing I haven't sent v2 yet; I'm making more edits to drop
the export flag, and require unconditional support for the command flag
once structured reads are negotiated.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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