Re: [Xen-devel] [PATCH] xen/arm: Register re-mapped Xen area as a temporary virtual region

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Konrad,

On 2017/3/14 21:56, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 14, 2017 at 01:32:31PM +0000, Julien Grall wrote:
>> Hi Konrad,
>>
>> On 14/03/17 13:20, Konrad Rzeszutek Wilk wrote:
>>> On Tue, Mar 14, 2017 at 05:27:17PM +0800, Wei Chen wrote:
>>>> While we're doing apply_alternatives, we will generate new instructions
>>>> if required. The new instructions need to update the Xen text section,
>>>> but Xen text section is read-only. So we re-map Xen to a new virtual
>>>> address to enable write access.
>>>>
>>>> The targets of the new generated instructions are located in this
>>>> re-mapped Xen area. But we haven't register this area as a virtual
>>>> region, so the checking code determines the targets are not in the
>>>> Xen text section, the new instructions could not be generated.
>>>
>>> Could you expand on that please? Where is the checking code that determines
>>> this? Are we talking about the traps handling and them scanning this
>>> new region?
>>>
>>> But you are saying 'new instructions'.. Hm, please enlighten!
>>
>> He is talking about the check in branch_insn_requires_update. There is some
>
> <lightbulb lights up>
>
>> sanity checking about the branch offset. Because Xen text section is marked
>> as read-only and we configure the hardware to not allow a region to be
>> writable and executable at the same time, we need to re-map Xen in a
>> temporary area. This means that the pc given in parameter of
>> branch_insn_requires_update will point to a value in the re-mapped area. So
>> the check is_active_kernel_text will always return false.
>
> Yup.
>>
>> Registering the virtual region temporarily will solve the problem.
>>
>> Wei, it would be worth to explain that you hit the BUG(); in branch_insn_*
>> and show an example in the commit message.
>
> <nods>

Thanks for your comments, I sent this patch to quickly and hadn't
provided enough notes. I will update the commit message in the next
version.

>>
>> Cheers,
>>
>> --
>> Julien Grall
>


-- 
Regards,
Wei Chen

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