Re: [4.17?] Re: [PATCH] common: map_vcpu_info() wants to unshare the underlying page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.10.2022 14:22, Julien Grall wrote:
> On 26/10/2022 03:03, Henry Wang wrote:
>>> -----Original Message-----
>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>> Sent: Wednesday, October 26, 2022 12:07 AM
>>> To: Henry Wang <Henry.Wang@xxxxxxx>
>>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper
>>> <andrew.cooper3@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>;
>>> Julien Grall <julien@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>;
>>> Wei Liu <wl@xxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> Subject: [4.17?] Re: [PATCH] common: map_vcpu_info() wants to unshare
>>> the underlying page
>>>
>>> On 25.10.2022 17:42, Roger Pau Monné wrote:
>>>> On Tue, Oct 11, 2022 at 10:48:38AM +0200, Jan Beulich wrote:
>>>>> Not passing P2M_UNSHARE to get_page_from_gfn() means there won't
>>> even be
>>>>> an attempt to unshare the referenced page, without any indication to the
>>>>> caller (e.g. -EAGAIN). Note that guests have no direct control over
>>>>> which of their pages are shared (or paged out), and hence they have no
>>>>> way to make sure all on their own that the subsequent obtaining of a
>>>>> writable type reference can actually succeed.
>>>>>
>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>
>>>> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>
>>> I didn't Cc you on the initial submission because mem-sharing isn't a
>>> supported feature, but upon reconsideration I thought I'd at least ask
>>> whether you would want to give this a release-ack. I don't really see
>>> any risk associated with it.
>>
>> By looking at the patch itself, this change seems ok to me, so I think
>> I will not block it, but I think Arm maintainers' approval might be needed
>> because of the discussion in [1], so I added them for their information.
>> If Arm maintainers do not object the change, you can have my release-ack.

I'll take the liberty then to translate this into an actual tag, with ...

> The P2M query type is so far ignored on Arm as we neither support 
> populate-on-demand nor memsharing.
> 
> I am assuming that if we ever introduce any those of features, we would 
> follow the same behavior as x86. So:
> 
> Acked-by: Julien Grall <jgrall@xxxxxxxxxx>

... this now in place (thanks Julien).

Jan