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Date:   Fri, 14 Aug 2020 14:54:38 +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 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.

In case you are worrying about gnt devices, I'd be fine to switch to

default XEN_BACKEND || XEN_GNTDEV


Juergen

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