From: Julien Grall <julien.grall@arm.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Wei Chen <Wei.Chen@arm.com>
Cc: Kaly.Xin@arm.com, nd@arm.com, sstabellini@kernel.org,
xen-devel@lists.xen.org, steve.capper@arm.com
Subject: Re: [PATCH] xen/arm: Register re-mapped Xen area as a temporary virtual region
Date: Tue, 14 Mar 2017 13:32:31 +0000 [thread overview]
Message-ID: <a3f8c517-6685-90ba-9653-1d3f8f8a9745@arm.com> (raw)
In-Reply-To: <20170314132039.GB472@char.us.oracle.com>
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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next prev parent reply other threads:[~2017-03-14 13:32 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-14 9:27 [PATCH] xen/arm: Register re-mapped Xen area as a temporary virtual region Wei Chen
2017-03-14 13:20 ` Konrad Rzeszutek Wilk
2017-03-14 13:32 ` Julien Grall [this message]
2017-03-14 13:56 ` Konrad Rzeszutek Wilk
2017-03-15 6:53 ` Wei Chen
2017-03-15 6:48 ` Wei Chen
2017-03-14 15:35 ` Julien Grall
2017-03-15 6:52 ` Wei Chen
2017-03-14 16:01 ` Julien Grall
2017-03-15 6:54 ` Wei Chen
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