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

[Paulina Szubarczyk <paulinaszubarczyk@gmail.com>]

On Mon, 2016-06-13 at 11:58 +0100, David Vrabel wrote:
> 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.

I think then there should be possibility for setting this in 
xl block-attach <disk-spec-component(s)> and in the config file that 
defines the domain in the corresponding fields. But if there is 
a need for such feature I would rather do it in a different patch.

Paulina


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