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

[Julien Grall <julien.grall@xxxxxxx>]

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...
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.
Cheers.

--
Julien Grall

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