Re: [Xen-devel] [PATCH 06/14 v4] xen/arm: vpl011: Add a new domctl API to initialize vpl011

[Bhupinder Thakur <bhupinder.thakur@xxxxxxxxxx>]

Hi Wei,

On 19 June 2017 at 17:17, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> On Mon, Jun 19, 2017 at 12:01:32PM +0100, Julien Grall wrote:
>>
>>
>> On 19/06/17 11:59, Bhupinder Thakur wrote:
>> > Hi Julien,
>> >
>> > I was mistaken in my earlier mail about vpl011 init working if it is
>> > moved to libxl__arch_domain_create(). It is failing because as you
>> > have mentioned vuart_pfn is allocated later in xc_dom_build_image().
>> >
>> > Can we delay mapping of this page in Xen until the ring buffer is
>> > actually required by the emulation code for reading/writing data. By
>> > that time, the page would have been physically mapped.
>>
>> You would not be able to report an error if you fail to map it. But this
>> looks like to me a workaround for a tool problem.
>>
>> Anyway, as I said, I'd like feedback from the tools maintainers to see how
>> we can proceed.
>>
>
> Is there a summary of the problem, is there a particular email in this
> thread I should look at? Sorry I'm swamped by emails and patches at the
> moment.

I will summarize the problem.

It was decided to call domain_vpl011_init() from inside
libxl__arch_domain_create() to initialize vpl011. However,
domain_vpl011_init() fails to map the the vuart GFN because it has not
been physically mapped yet by the toolstack.

The following call flows highlights the issue.

libxl__domain_build() ---> libxl__build_pv ---> libxl__build_dom()
----> xc_dom_build_image() ---> alloc_magic_pages() ----> vuart GFN
allocated/mapped here

libxl__domain_build() ----> libxl__build_pre()  ---->
libxl__arch_domain_create() ----> domain_vpl011_init() ---> this call
fails as the vuart GFN has not been physically mapped yet as shown in
the first call flow.

However, libxl__build_pv() is called after libxl__build_pre(). It
means that the domain_vpl011_init() is called before
alloc_magic_pages() is called and hence the initialization fails.

For that reason, I had introduced a new function
libxl__arch_domain_create_finish() which will be called from
libxl__build_post(). I moved the domain_vpl011_init() there. However,
Julien pointed out that vuart should be initialized early in
libxl__arch_domain_create() function.

So the issue is what is the right place to call domain_vpl011_init()?

I hope it clarifies the issue.

Regards,
Bhupinder

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