Re: [kvm-unit-tests PATCH v3 00/27] EFI and ACPI support for arm64

Alexandru Elisei
2022-08-10T15:03:37Z

Hi Sean,

On Wed, Aug 10, 2022 at 02:58:15PM +0000, Sean Christopherson wrote:
> On Wed, Aug 10, 2022, Alexandru Elisei wrote:
> > Hi Sean,
> >
> > On Tue, Aug 09, 2022 at 03:29:58PM +0000, Sean Christopherson wrote:
> > > On Tue, Aug 09, 2022, Alexandru Elisei wrote:
> > > > Note that the assumption that efi_main() makes is that setup_efi() doesn't
> > > > change the stack from the stack that the UEFI implementation allocated, in
> > > > order for setup_efi() to be able to return to efi_main().
> > >
> > > On the x86 side, efi_main() now runs with a KUT-controlled stack since commit
> > >
> > > d316d12a ("x86: efi: Provide a stack within testcase memory")
> > >
> > > > If we want to keep the UEFI allocated stack, then both mechanism must be
> > > > forbidden when running under UEFI. I dislike this idea, because those two
> > > > mechanisms allow kvm-unit-tests to run tests which otherwise wouldn't have
> > > > been possible with a normal operating system, which, except for the early
> > > > boot code, runs with the MMU enabled.
> > >
> > > Agreed. IMO, KUT should stop using UEFI-controlled data as early as possible.
> > > The original x86 behavior was effectively a temporary solution to get UEFI working
> > > without needing to simultaneously rework the common early boot flows.
> >
> > Yes, this is also what I am thinking, the stack is poorly specified in the
> > specification because the specification doesn't expect an application to
> > keep using it after calling EFI_BOOT_SERVICES.Exit(). Plus, using the UEFI
> > allocated stack makes the test less reproducible, as even EDK2 today
> > diverges from the spec wrt the stack, and other UEFI implementations might
> > do things differently. And with just like all software, there might be bugs
> > in the firmware. IMO, the more control kvm-unit-tests has over its
> > resources, the more robust the tests are.
> >
> > What I was thinking is rewriting efi_main to return setup_efi(),
> > something like this:
> >
> > void efi_main(efi_handle_t handle, efi_system_table_t *sys_tab)
> > {
> > /* Get image, cmdline and memory map parameters from UEFI */
> >
> > efi_exit_boot_services(handle, &efi_bootinfo.mem_map);
> >
> > /* Set up arch-specific resources, not expected to return. */
> > setup_efi(&efi_bootinfo);
> > }
> >
> > Which would allow all architectures to change their environment as they see
> > fit, as setup_efi() is not expected to return. Architectures would have to
> > be made aware of the efi_exit() function though.
>
> And they'd also have to invoke the test's main(). Why can't ARM switch to a KUT
> stack before calling efi_main()? The stack is a rather special case, I don't think
> it's unreasonable to require architectures to handle that before efi_main().

Sorry, missed the part from your earlier reply about x86 switching to a
kvm-unit-tests controlled stack. I was just going to say that, when I saw
your reply.

I agree, it's entirely feasible for arm64 to switch to its own stack before
calling efi_main(), in which case it can call efi_main() without any
changes to efi_main().

Thanks for your input.

Alex