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Re: [PATCH v3 14/19] xen/arm: add Persistent Map (PMAP) infrastructure
- To: Julien Grall <julien@xxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 28 Feb 2022 11:30:07 +0100
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- Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Hongyan Xia <hongyxia@xxxxxxxxxx>, Julien Grall <jgrall@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 28 Feb 2022 10:30:24 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 28.02.2022 11:20, Julien Grall wrote:
> On 28/02/2022 10:10, Jan Beulich wrote:
>> On 28.02.2022 10:55, Julien Grall wrote:
>>> On 22/02/2022 15:22, Jan Beulich wrote:
>>>> On 21.02.2022 11:22, Julien Grall wrote:
>>>>> + /*
>>>>> + * We cannot use set_fixmap() here. We use PMAP when there is no
>>>>> direct map,
>>>>> + * so map_pages_to_xen() called by set_fixmap() needs to map pages on
>>>>> + * demand, which then calls pmap() again, resulting in a loop.
>>>>> Modify the
>>>>> + * PTEs directly instead. The same is true for pmap_unmap().
>>>>> + */
>>>>> + arch_pmap_map(slot, mfn);
>>>>
>>>> I'm less certain here, but like above I'm under the impression
>>>> that this comment may no longer be accurate.
>>>
>>> This comment is still accurate for Arm. I also expect it to be accurate
>>> for all architectures because set_fixmap() is likely going to be
>>> implemented with generic PT helpers.
>>>
>>> So I think it makes sense to keep it in common code. This explains why
>>> we are calling arch_pmap_map() rather than set_fixmap() directly.
>>
>> I guess I was rather after "when there is no direct map" alluding to the
>> planned removal of it on x86.
> It is one way to interpret it. The other way is that pmap will be used
> when the directmap is not yet in place.
>
> But I guess I could be less specific and refers to the fact the domain
> page infrastructure is not yet setup. Would that be better for you?
That or simply add "(yet)" to what is there.
Jan
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