Re: [Xen-devel] [PATCH v2 14/25] arm/altp2m: Make get_page_from_gva ready for altp2m.

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

Hi Julien,


On 08/06/2016 03:45 PM, Julien Grall wrote:
>
>
> On 06/08/2016 11:38, Sergej Proskurin wrote:
>> Hi Julien,
>
> Hello Serge,
>
>> On 08/04/2016 01:59 PM, Julien Grall wrote:
>>> Hello Sergej,
>>>
>>> On 01/08/16 18:10, Sergej Proskurin wrote:
>>>> The function get_page_from_gva uses ARM's hardware support to
>>>> translate
>>>> gva's to machine addresses. This function is used, among others, for
>>>> memory regulation purposes, e.g, within the context of memory
>>>> ballooning.
>>>> To ensure correct behavior while altp2m is in use, we use the
>>>> host's p2m
>>>> table for the associated gva to ma translation. This is required at
>>>> this
>>>> point, as altp2m lazily copies pages from the host's p2m and even
>>>> might
>>>> be flushed because of changes to the host's p2m (as it is done within
>>>> the context of memory ballooning).
>>>
>>> I was expecting to see some change in
>>> p2m_mem_access_check_and_get_page. Is there any reason to not fix it?
>>>
>>>
>>
>> I did not yet encounter any issues with
>> p2m_mem_access_check_and_get_page. According to ARM ARM, ATS1C** (see
>> gva_to_ipa_par) translates VA to IPA in non-secure privilege levels (as
>> it is the the case here). Thus, the 2nd level translation represented by
>> the (alt)p2m is not really considered at this point and hence make an
>> extension obsolete.
>>
>> Or did you have anything else in mind?
>
> The stage-1 page tables are living in the guest memory. So every time
> you access an entry in the page table, you have to translate the IPA
> (guest physical address) into a PA.
>
> However, the underlying memory of those page table may have
> restriction permission or does not exist in the altp2m at all. So the
> translation will fail.
>

Please correct me if I am wrong but as far as I understand: the function
p2m_mem_access_check_and_get_page is called only from get_page_from_gva.
Also it is called only if the page translation within the function
get_page_from_gva was not successful. Because of the fact that we use
the hostp2m's 2nd stage translation table including the original memory
access permissions (please note the short sequence, where we temporarily
reset the VTTBR_EL2 of the hostp2m if altp2m is active), potential
faults (which would lead to the call of the function
p2m_mem_access_check_and_get_page) must have reasons beyond altp2m.

Best regards,
~Sergej



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