Re: [PATCH v2 2/2] qdisk - hw/block/xen_disk: grant copy implementation

[David Vrabel <david.vrabel@citrix.com>]

On 13/06/16 11:44, Paulina Szubarczyk wrote:
> On Mon, 2016-06-13 at 11:15 +0100, David Vrabel wrote:
>> On 13/06/16 10:43, Paulina Szubarczyk wrote:
>>> Copy data operated on during request from/to local buffers to/from 
>>> the grant references. 
>>>
>>> Before grant copy operation local buffers must be allocated what is 
>>> done by calling ioreq_init_copy_buffers. For the 'read' operation, 
>>> first, the qemu device invokes the read operation on local buffers 
>>> and on the completion grant copy is called and buffers are freed. 
>>> For the 'write' operation grant copy is performed before invoking 
>>> write by qemu device. 
>>>
>>> A new value 'feature_grant_copy' is added to recognize when the 
>>> grant copy operation is supported by a guest. 
>>> The body of the function 'ioreq_runio_qemu_aio' is moved to 
>>> 'ioreq_runio_qemu_aio_blk' and in the 'ioreq_runio_qemu_aio' depending
>>> on the support for grant copy according checks, initialization, grant 
>>> operation are made, then the 'ioreq_runio_qemu_aio_blk' function is 
>>> called. 
>>
>> I think you should add an option to force the use of grant mapping even
>> if copy support is detected.  If future changes to the grant map
>> infrastructure makes it faster or if grant map scales better in some
>> systems, then it would be useful to be able to use it.
> 
> The 'feature_grant_copy' is a boolean and could be set to false in such case.
> There could be added a node in XenStore, for example
> 'feature-force-grant-map', which when set by frontend will be read
> during a connection and changed the value to false forcing the grant map
> operation.

This option should not be exposed/controlled by the frontend.  I was
thinking of a command line option to qemu or similar.

David

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