Re: [Xen-devel] [PATCH v2 3/5] tools/libxendevicemodel: extract functions and add a compat layer

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/02/17 15:04, Ian Jackson wrote:
> Andrew Cooper writes ("Re: [Xen-devel] [PATCH v2 3/5] 
> tools/libxendevicemodel: extract functions and add a compat layer"):
>> On 22/02/17 14:37, Ian Jackson wrote:
>>> Is the bitmap copied in each dmop call ?
>> Yes, and hence...
> Oh!
>
> I had assumed that this wouldn't be the case because it would
> obviously be slow.
>
>> ... one optimisation I want to do in the future is for the DM to be able
>> to make a Read-only mapping of Xens logdirty bitmap, so the bitmap can
>> be passed by shared memory rather than memcpy(), because I expect it
>> would be rather more efficient during some of the time-sensitive bits of
>> live migrate.
> Well, err, yes.
>
> (What about the ioreq ring?)

That is a piece of shared memory.

But shared memory is not interesting to DMOP.  You already *must* defer
to your kernel to create the shared memory in the first place, so the
resulting pointer is definitely ok as far as the kernel is concerned.

~Andrew

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