Re: Xen on RP4

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 29 Oct 2020, Jürgen Groß wrote:
> On 29.10.20 01:37, Stefano Stabellini wrote:
> > On Tue, 27 Oct 2020, Elliott Mitchell wrote:
> > > On Mon, Oct 26, 2020 at 06:44:27PM +0000, Julien Grall wrote:
> > > > On 26/10/2020 16:03, Elliott Mitchell wrote:
> > > > > On Mon, Oct 26, 2020 at 01:31:42PM +0000, Julien Grall wrote:
> > > > > > On 24/10/2020 06:35, Elliott Mitchell wrote:
> > > > > > > ACPI has a distinct
> > > > > > > means of specifying a limited DMA-width; the above fails, because
> > > > > > > it
> > > > > > > assumes a *device-tree*.
> > > > > > 
> > > > > > Do you know if it would be possible to infer from the ACPI static
> > > > > > table
> > > > > > the DMA-width?
> > > > > 
> > > > > Yes, and it is.  Due to not knowing much about ACPI tables I don't
> > > > > know
> > > > > what the C code would look like though (problem is which documentation
> > > > > should I be looking at first?).
> > > > 
> > > > What you provided below is an excerpt of the DSDT. AFAIK, DSDT content
> > > > is written in AML. So far the shortest implementation I have seen for
> > > > the AML parser is around 5000 lines (see [1]). It might be possible to
> > > > strip some the code, although I think this will still probably too big
> > > > for a single workaround.
> > > > 
> > > > What I meant by "static table" is a table that looks like a structure
> > > > and can be parsed in a few lines. If we can't find on contain the DMA
> > > > window, then the next best solution is to find a way to identity the
> > > > platform.
> > > > 
> > > > I don't know enough ACPI to know if this solution is possible. A good
> > > > starter would probably be the ACPI spec [2].
> > > 
> > > Okay, that is worse than I had thought (okay, ACPI is impressively
> > > complex for something nominally firmware-level).
> > > 
> > > There are strings in the present Tianocore implementation for Raspberry
> > > PI 4B which could be targeted.  Notably included in the output during
> > > boot listing the tables, "RPIFDN", "RPIFDN RPI" and "RPIFDN RPI4" (I'm
> > > unsure how kosher these are as this wsn't implemented nor blessed by the
> > > Raspberry PI Foundation).
> > > 
> > > I strongly dislike this approach as you soon turn the Xen project into a
> > > database of hardware.  This is already occurring with
> > > xen/arch/arm/platforms and I would love to do something about this.  On
> > > that thought, how about utilizing Xen's command-line for this purpose?
> > 
> > I don't think that a command line option is a good idea: basically it is
> > punting to users the task of platform detection. Also, it means that
> > users will be necessarily forced to edit the uboot script or grub
> > configuration file to boot.
> > 
> > Note that even if we introduced a new command line, we wouldn't take
> > away the need for xen/arch/arm/platforms anyway.
> > 
> > It would be far better for Xen to autodetect the platform if we can.
> > 
> > 
> > > Have a procedure of during installation/updates retrieve DMA limitation
> > > information from the running OS and the following boot Xen will follow
> > > the appropriate setup.  By its nature, Domain 0 will have the information
> > > needed, just becomes an issue of how hard that is to retrieve...
> > 
> > Historically that is what we used to do for many things related to ACPI,
> > but unfortunately it leads to a pretty bad architecture where Xen
> > depends on Dom0 for booting rather than the other way around. (Dom0
> > should be the one requiring Xen for booting, given that Xen is higher
> > privilege and boots first.)
> > 
> > 
> > I think the best compromise is still to use an ACPI string to detect the
> > platform. For instance, would it be possible to use the OEMID fields in
> > RSDT, XSDT, FADT?  Possibly even a combination of them?
> > 
> > Another option might be to get the platform name from UEFI somehow.
> 
> What about having a small domain parsing the ACPI booting first and use
> that information for booting dom0?
> 
> That dom would be part of the Xen build and the hypervisor wouldn't need
> to gain all the ACPI AML logic.

That could work, but in practice we don't have such a domain today --
the infrastructure is missing. I wonder whether the bootloader (uboot or
grub) would know about the platform and might be able to pass that
information to Xen somehow.