Re: [Xen-devel] [PATCH v5 09/12] arm/mem_access: Add software guest-page-table walk

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

Hi Julien,

On 07/04/2017 06:58 PM, Julien Grall wrote:
> Hi Sergej,
> 
> On 06/27/2017 12:52 PM, Sergej Proskurin wrote:
>> The function p2m_mem_access_check_and_get_page in mem_access.c
>> translates a gva to an ipa by means of the hardware functionality of the
>> ARM architecture. This is implemented in the function gva_to_ipa. If
>> mem_access is active, hardware-based gva to ipa translation might fail,
>> as gva_to_ipa uses the guest's translation tables, access to which might
>> be restricted by the active VTTBR. To address this issue, in this commit
>> we add a software-based guest-page-table walk, which will be used by the
>> function p2m_mem_access_check_and_get_page perform the gva to ipa
>> translation in software in one of the following commits.
>>
>> Note: The introduced function guest_walk_tables assumes that the domain,
>> the gva of which is to be translated, is running on the currently active
>> vCPU. To walk the guest's page tables on a different vCPU, the following
>> registers would need to be loaded: TCR_EL1, TTBR0_EL1, TTBR1_EL1, and
>> SCTLR_EL1.
>>
>> Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
>> Acked-by: Julien Grall <julien.grall@xxxxxxx>
>> ---
>> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> Cc: Julien Grall <julien.grall@xxxxxxx>
>> ---
>> v2: Rename p2m_gva_to_ipa to p2m_walk_gpt and move it to p2m.c.
>>
>>      Move the functionality responsible for walking long-descriptor based
>>      translation tables out of the function p2m_walk_gpt. Also move out
>>      the long-descriptor based translation out of this commit.
>>
>>      Change function parameters in order to return access access rights
>>      to a requested gva.
>>
>>      Cosmetic fixes.
>>
>> v3: Rename the introduced functions to guest_walk_(tables|sd|ld) and
>>      move the implementation to guest_copy.(c|h).
>>
>>      Set permissions in guest_walk_tables also if the MMU is disabled.
>>
>>      Change the function parameter of type "struct p2m_domain *" to
>>      "struct vcpu *" in the function guest_walk_tables.
>>
>> v4: Change the function parameter of type "struct p2m_domain *" to
>>      "struct vcpu *" in the functions guest_walk_(sd|ld) as well.
>>
>> v5: Merge two if-statements in guest_walk_tables to ease readability.
>>
>>      Set perms to GV2M_READ as to avoid undefined permissions.
> 
> I would appreciate if you mention that you kept my tag even with the
> change made and asked whether I am happy with it...
> 

Sorry for not mentioning that. Since it was your suggestion right after
you have acked this patch, I thought it would be sufficient to just
state the changes. I will consider that in the future.

> In this case, you need a bit more rationale to explain why setting to
> GV2M_READ by default is fine... If it is just a random value, then say it.
> 

I will add a comment in the change log stating that we set perms to
GV2M_READ (that equals to 0) by default to avoid having potentially
random values returned on errors.

Shall I remove your ack for this patch in the next version?

Thanks,
~Sergej

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