linux-kernel - Re: [PATCH v4 2/2] xen: add helpers to allocate unpopulated memory

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Date:   Fri, 14 Aug 2020 15:52:39 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>, Wei Liu <wl@....org>,
        Yan Yankovskyi <yyankovskyi@...il.com>,
        dri-devel@...ts.freedesktop.org, xen-devel@...ts.xenproject.org,
        linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v4 2/2] xen: add helpers to allocate unpopulated memory

On 14.08.20 15:35, Roger Pau Monné wrote:
> On Fri, Aug 14, 2020 at 02:54:38PM +0200, Jürgen Groß wrote:
>> On 14.08.20 14:47, Roger Pau Monné wrote:
>>> On Fri, Aug 14, 2020 at 12:27:32PM +0200, Jürgen Groß wrote:
>>>> On 14.08.20 11:56, Roger Pau Monné wrote:
>>>>> On Fri, Aug 14, 2020 at 08:29:20AM +0100, Christoph Hellwig wrote:
>>>>>> On Thu, Aug 13, 2020 at 09:54:20AM +0200, Roger Pau Monn?? wrote:
>>>>>>> On Thu, Aug 13, 2020 at 08:33:37AM +0100, Christoph Hellwig wrote:
>>>>>>>> On Tue, Aug 11, 2020 at 11:44:47AM +0200, Roger Pau Monne wrote:
>>>>>>>>> If enabled (because ZONE_DEVICE is supported) the usage of the new
>>>>>>>>> functionality untangles Xen balloon and RAM hotplug from the usage of
>>>>>>>>> unpopulated physical memory ranges to map foreign pages, which is the
>>>>>>>>> correct thing to do in order to avoid mappings of foreign pages depend
>>>>>>>>> on memory hotplug.
>>>>>>>>
>>>>>>>> So please just select ZONE_DEVICE if this is so much better rather
>>>>>>>> than maintaining two variants.
>>>>>>>
>>>>>>> We still need to other variant for Arm at least, so both need to be
>>>>>>> maintained anyway, even if we force ZONE_DEVICE on x86.
>>>>>>
>>>>>> Well, it still really helps reproducability if you stick to one
>>>>>> implementation of x86.
>>>>>>
>>>>>> The alternative would be an explicit config option to opt into it,
>>>>>> but just getting a different implementation based on a random
>>>>>> kernel option is strange.
>>>>>
>>>>> Would adding something like the chunk below to the patch be OK?
>>>>>
>>>>> ---8<---
>>>>> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
>>>>> index 018020b91baa..5f321a1319e6 100644
>>>>> --- a/drivers/xen/Kconfig
>>>>> +++ b/drivers/xen/Kconfig
>>>>> @@ -328,7 +328,14 @@ config XEN_FRONT_PGDIR_SHBUF
>>>>>     	tristate
>>>>>     config XEN_UNPOPULATED_ALLOC
>>>>> -	bool
>>>>> -	default y if ZONE_DEVICE && !ARM && !ARM64
>>>>> +	bool "Use unpopulated memory ranges for guest mappings"
>>>>> +	depends on X86
>>>>> +	select ZONE_DEVICE
>>>>> +	default y
>>>>
>>>> I'd rather use "default XEN_BACKEND" here, as mappings of other guest's
>>>> memory is rarely used for non-backend guests.
>>>
>>> There's also the privcmd and gnt devices which make heavy use of this,
>>> so I'm not sure only selecting by default on XEN_BACKEND is the best
>>> option.
>>
>> I just want to avoid that kernels built for running as Xen guest, but
>> not as dom0, will be forced to select ZONE_DEVICE.
>>
>> As privcmd is dom0-only, this is no problem.
> 
> Oh, didn't know that, I somehow assumed privcmd would be available to
> all Xen guests regardless of whether dom0 support was selected.

My remark above should have been more precise in this regard:

privcmd operations dealing with foreign mappings are normally limited
to dom0 (there might be special cases, like linux-based stubdoms, but
those will be configured carefully for that purpose, so they don't
need to be considered for selecting the default IMO).

> 
>> In case you are worrying about gnt devices, I'd be fine to switch to
>>
>> default XEN_BACKEND || XEN_GNTDEV
> 
> Sure. maybe even:
> 
> default XEN_BACKEND || XEN_GNTDEV || XEN_DOM0

Yes.

> 
> Do you want me to resend with this changed or are you happy to fixup
> if there are no further comments?

I'd prefer a resend (maybe after Patch 1 has gained its missing Ack, and
then with Patch 1 sent to me, too).


Juergen