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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Jun 20, 2017 at 12:16:52PM +0100, Julien Grall wrote:
> On 06/19/2017 02:11 PM, Bhupinder Thakur wrote:
> > Hi Wei,
> 
> Hi Bhupinder,
> 
> > 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.
> 
> libxl__arch_domain_create could be a place or even
> libxl__arch_domain_finalise_hw_descriptions.
> 

libxl__arch_domain_finialise_hw_descriptions sounds like a good place to
me.

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