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

[Julien Grall <julien.grall@xxxxxxx>]

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 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.
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.
Cheers,

--
Julien Grall

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