Re: [Qemu-block] [PATCH] proto: add 'shift' extension.

[Eric Blake]

On 09/26/2016 07:51 AM, Paolo Bonzini wrote:

>> +- bit 3, `NBD_CMD_FLAG_SHIFT`; This flag is valid for `NBD_CMD_TRIM` and
>> +  `NBD_CMD_WRITE_ZEROES`. If this flag is set the server shifts request
>> +  *length* and *offset* left by N bits, where N is defined by 
>> `NBD_OPT_SHIFT`
>> +  option or is assumed to be 16 bits by default if `NBD_OPT_SHIFT` option is
>> +  not specified. If after shift `(offset + length)` exceeds disk size, 
>> length
>> +  should be reduced to `(<disk size> - offset)`. However, `(offset + 
>> length)`
>> +  must not exceed disk size by more than `(1 << N) - 1`.
>>  
>>  #### Request types
>>  
>>
> 
> This is very ad hoc.  Can we instead have a block size common to all
> commands?  Block devices in practice have one, in fact that's why
> they're called block devices...

The INFO extensions are supposed to be a way for the server to
communicate block sizes to the guest (note, it is one-way communication;
the guest does NOT get to pick an arbitrary size, but relies on the
server reporting it) - but I don't know if that sizing information is
useful enough for the task at hand.  As it was, the INFO extension had a
proposal idea a while back about advertising a different size for TRIM
and WRITE_ZEROES than what is preferred for WRITE and READ, so having a
single size that works for shifted commands that operate on blocks
instead of bytes may not even be feasible if there is no single block
size to settle on.

But having a universal command flag that says "this command requests
offset and length in units of blocks instead of bytes", where blocks is
an up-front sizing settled on during handshake via INFO extension, and
NOT something that the client can change at-will during a live session,
may be useful.  Or it may be the source of arithmetic overflow exploits
in poor implementations, or of denial-of-service with used with a READ
or WRITE to send more than 2G of data in a single command.  In other
words, I don't yet see a compelling use case for being able to request
shifted sizes.

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

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