ARM: KVM/XEN: how should we support virt-what?

[Andrew Jones]

On Thu, Mar 26, 2015 at 07:50:06PM +0100, Ard Biesheuvel wrote:
> On 26 March 2015 at 19:49, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> > On 26 March 2015 at 19:45, Stefano Stabellini
> > <stefano.stabellini at eu.citrix.com> wrote:
> >> On Thu, 26 Mar 2015, Andrew Jones wrote:
> >>> On Wed, Mar 25, 2015 at 10:44:42AM +0100, Andrew Jones wrote:
> >>> > Hello ARM virt maintainers,
> >>> >
> >>> > I'd like to start a discussion about supporting virt-what[1]. virt-what
> >>> > allows userspace to determine if the system it's running on is running
> >>> > in a guest, and of what type (KVM, Xen, etc.). Despite it being a best
> >>> > effort tool, see the Caveat emptor in [1], it has become quite a useful
> >>> > tool, and is showing up in different places, such as OpenStack. If you
> >>> > look at the code[2], specifically [3], then you'll see how it works on
> >>> > x86, which is to use the dedicated hypervisor cpuid leaves. I'm
> >>> > wondering what equivalent we have, or can develop, for arm.
> >>> > Here are some thoughts;
> >>> > 0) there's already something we can use, and I just need to be told
> >>> >    about it.
> >>> > 1) be as similar as possible to x86 by dedicating some currently
> >>> >    undefined sysreg bits. This would take buy-in from lots of parties,
> >>> >    so is not likely the way to go.
> >>> > 2) create a specific DT node that will get exposed through sysfs, or
> >>> >    somewhere.
> >>> > 3) same as (2), but just use the nodes currently in mach-virt's DT
> >>> >    as the indication we're a guest. This would just be a heuristic,
> >>> >    i.e. "have virtio mmio" && psci.method == hvc, or something,
> >>> >    and we'd still need a way to know if we're kvm vs. xen vs. ??.
> >>> >
> >>> > Thanks,
> >>> > drew
> >>> >
> >>> > [1] http://people.redhat.com/~rjones/virt-what/
> >>> > [2] http://git.annexia.org/?p=virt-what.git;a=summary
> >>> > [3] http://git.annexia.org/?p=virt-what.git;a=blob_plain;f=virt-what-cpuid-helper.c;hb=HEAD
> >>>
> >>> Thanks everyone for their responses. So, the current summary seems to
> >>> be;
> >>> 1) Xen has both a DT node and an ACPI table, virt-what can learn how
> >>>    to probe those.
> >>> 2) We don't have anything yet for KVM, and we're reluctant to create a
> >>>    specific DT node. Anyway, we'd still need to address ACPI booted
> >>>    guests some other way.
> >>>
> >>> For a short-term, DT-only, approach we could go with a heuristic, one
> >>> that includes Marc's "if hypervisor node exists, then xen, else kvm"
> >>> condition.
> >>>
> >>> How about SMBIOS for a long-term solution that works for both DT and
> >>> ACPI? We're not populating SMBIOS for arm guests yet in qemu, but now
> >>> that AAVMF has fw_cfg, we should be able to. On x86 we already have
> >>> smbios populated from qemu, although not in a way that allows us to
> >>> determine kvm vs. xen vs. tcg.
> >>
> >> I don't think that SMBIOS works with DT.
> >>
> >
> > SMBIOS works fine with DT
> 
> ... but it needs UEFI ...

Yes. Perhaps the short-term solution will be a long-term solution for
DT-only, non-UEFI guests, but we can do better for the guests with UEFI,
that may or may not be ACPI, by using SMBIOS.

Unless somebody objects to either/both of these paths, then I guess
we'll start heading down them both.

Thanks,
drew