Re: [Qemu-devel] [Linaro-acpi] [RFC PATCH 0/7] hw/arm/virt: Dynamic ACPI v5.1 table generation

[Mark Rutland <mark.rutland@arm.com>]

Hi Christoffer,

On Tue, Nov 11, 2014 at 04:31:01PM +0000, Christoffer Dall wrote:
> On Tue, Nov 11, 2014 at 03:29:33PM +0000, Mark Rutland wrote:
> > Hi,
> > 
> > On Thu, Nov 06, 2014 at 01:33:20PM +0000, Alexander Spyridakis wrote:
> > > On 6 November 2014 14:44, Peter Maydell <peter.maydell@linaro.org> wrote:
> > > >
> > > >
> > > > > We need ACPI guest support in QEMU for AArch64 over here, with all features
> > > > > (including the ability to run ACPI code and add specific tables), for
> > > > > ACPI-based guests.
> > > >
> > > > The plan for providing ACPI to guests is that we run a UEFI BIOS
> > > > blob which is what is responsible for providing ACPI and UEFI
> > > > runtime services to guests which need them. (The UEFI blob finds
> > > > out about its hardware by looking at a device tree that QEMU
> > > > passes it, but that's a detail between QEMU and its bios blob).
> > > > This pretty much looks like what x86 QEMU used to do with ACPI
> > > > for a very long time, so we know it's a feasible approach.
> > > 
> > > Hi Peter,
> > > 
> > > The rational in the proposed approach is meant for cases where the
> > > user does not want to rely on external firmware layers. While UEFI
> > > could do what you are describing, the point is to avoid this not so
> > > trivial overhead in the booting process. Especially in the case of
> > > thin guests, where another software dependency is undesired.
> > 
> > I'm not sure how you plan to use ACPI without UEFI, as there are several
> > pieces of information which ACPI misses, such as the memory map, which
> > must be discovered from UEFI. How do you intend to discover the memory
> > map without UEFI?
> > 
> > Additionally, with Linux and other generic OSs, the expectation is that
> > the ACPI tables are discovered via the UEFI system table. How do you
> > intend to discover the ACPI tables? Or other system information?
> 
> FWIW, Xen needs to pass the RDSP pointer along with a tiny DT containing
> the command line and memory information to Dom0 as well.

When you say "memory information", is that pointers to a UEFI memory
map, or memory nodes? The former should work for ACPI, but I don't think
the latter will. I think there's a need for some discussion regarding
the Dom0 boot flow for ACPI. Is there any tree I can take a peek at?

Passing just the RDSP will mean that Dom0 won't get SMBIOS tables and
other potentially useful things, in addition to simply being yet another
potential boot configuration. I'm a little concerned about that.

> We are currently suggesting adding an RDSP property to the chosen node
> in the tiny DT, but a command-line arguement like kexec proposed could
> be another option I guess, albeit not a very pretty one.

I'm not sure what an RDSP command line property would have to do with
kexec. I'll assume I've misunderstood something.

> But, what I hear from Huawei is that they don't want any DT and don't
> want any UEFI, so not sure how they plan on accomplishing that.

Indeed.

Thanks,
Mark.