Re: [PATCH V4 3/3] libxl/arm: Add handling of extended regions for DomU

[Oleksandr <olekstysh@gmail.com>]

On 06.10.21 00:36, Stefano Stabellini wrote:

Hi Stefano

> On Tue, 5 Oct 2021, Oleksandr wrote:
>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>
>>> The extended region (safe range) is a region of guest physical
>>> address space which is unused and could be safely used to create
>>> grant/foreign mappings instead of wasting real RAM pages from
>>> the domain memory for establishing these mappings.
>>>
>>> The extended regions are chosen at the domain creation time and
>>> advertised to it via "reg" property under hypervisor node in
>>> the guest device-tree. As region 0 is reserved for grant table
>>> space (always present), the indexes for extended regions are 1...N.
>>> If extended regions could not be allocated for some reason,
>>> Xen doesn't fail and behaves as usual, so only inserts region 0.
>>>
>>> Please note the following limitations:
>>> - The extended region feature is only supported for 64-bit domain
>>>     currently.
>>> - The ACPI case is not covered.
>>>
>>> ***
>>>
>>> The algorithm to choose extended regions for non-direct mapped
>>> DomU is simpler in comparison with the algorithm for direct mapped
>>> Dom0. As we have a lot of unused space above 4GB, provide single
>>> 2MB-aligned region from the second RAM bank taking into the account
>>> the maximum supported guest address space size and the amount of
>>> memory assigned to the guest. The maximum size of the region is 128GB.
>>> The minimum size is 64MB.
>>>
>>> Suggested-by: Julien Grall <jgrall@amazon.com>
>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
>>> Acked-by: Ian Jackson <iwj@xenproject.org>
>>> Reviewed-by: Michal Orzel <michal.orzel@arm.com>
>>> Tested-by: Michal Orzel <michal.orzel@arm.com>
>> I though a bit more on this and decided to make a patch more functional by
>> trying to also allocate extended region below 4GB, I think we could do with
>> it.
>> Actually if guest memory size is less than GUEST_RAM0_SIZE, we are able to
>> provide unused space. I have tested with with various guest memory sizes and
>> it worked fine. Also I decided to drop limit for maximum extended region size
>> (128GB), we don't apply this limit in Dom0 and I don't see why we need it
>> here, moreover the calculation became more obvious. I will drop all acks and
>> send updated version. Are there any objections?
> I am OK with it; it looks like you made good improvements. One caveat is
> that I volunteer to review again no problem,

Great, thank you.


> but we'll need a new ack
> from Ian Jackson to commit.

Yes, I know that.


-- 
Regards,

Oleksandr Tyshchenko