Re: [PATCH] efi: stub: override RT_PROP table supported mask based on EFI variable

[Ard Biesheuvel <ardb@kernel.org>]

On Tue, 9 Mar 2021 at 19:10, Rob Clark <robdclark@gmail.com> wrote:
>
> On Mon, Mar 8, 2021 at 7:22 PM Shawn Guo <shawn.guo@linaro.org> wrote:
> >
> > On Mon, Mar 08, 2021 at 02:34:48PM +0100, Ard Biesheuvel wrote:
> > > On Sun, 7 Mar 2021 at 12:02, Shawn Guo <shawn.guo@linaro.org> wrote:
> > > >
> > > > On Sat, Mar 06, 2021 at 12:35:19PM +0100, Ard Biesheuvel wrote:
> > > > > Allow EFI systems to override the set of supported runtime services
> > > > > declared via the RT_PROP table, by checking for the existence of a
> > > > > 'OverrideSupported' EFI variable of the appropriate size under the
> > > > > RT_PROP table GUID, and if it does, combine the supported mask using
> > > > > logical AND. (This means the override can only remove support, not
> > > > > add it back).
> > > > >
> > > > > Cc: Jeffrey Hugo <jhugo@codeaurora.org>,
> > > > > Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
> > > > > Cc: Shawn Guo <shawn.guo@linaro.org>
> > > > > Cc: Rob Clark <robdclark@gmail.com>
> > > > > Cc: Leif Lindholm <leif@nuviainc.com>
> > > > > Cc: linux-arm-msm@vger.kernel.org
> > > > >
> > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > >
> > > > Awesome, Ard!  On both Lenovo Yoga C630 and Flex 5G latops:
> > > >
> > > > Tested-by: Shawn Guo <shawn.guo@linaro.org>
> > > >
> > > > With 'OverrideSupported' EFI variable added from UEFI Shell, we can drop
> > > > 'efi=novamap' kernel cmdline and get around the broken poweroff runtime
> > > > services nicely.  Thanks!
> > > >
> > >
> > > Thanks for confirming.
> > >
> > > However, I am not going to merge this without some justification, and
> > > hopefully some input from other folks (Leif?)
> > >
> > > RTPROP already provides what we need on all platforms that use
> > > DtbLoader, and the patch for that is queued up for v5.12-rcX, with a
> > > cc:stable to v5.10. This allows any RT service to be marked as
> > > disabled, including SetVirtualAddressMap().
> > >
> > > So afaict, that means that this patch would be a special case for
> > > Flex5G, right?
> >
> > It's for all Snapdragon based laptops, as we need to disable
> > SetVirtualAddressMap runtime services on all of them.
> >
> > > So how are platforms such as this one going to load the
> > > DTB? If some loader will be involved (or even just GRUB),
> >
> > Yes, GRUB.
> >
> > > shouldn't it
> > > be that component that sets RTPROP like DtbLoader will, not the kernel
> > > itself.
> > >
> > > Btw I don't think ACPI boot is a use case here. I don't see a software
> > > framebuffer with no wifi support as a usage mode that justifies
> > > carrying EFI stub hacks for everyone.
> >
> > Okay.  I'm fine to carry it as an out-of-tree patch until someday you
> > consider ACPI boot is useful for everyone.  But I do boot these laptops
> > with ACPI at daily basis right now as arm64 native build machine, with
> > USB Ethernet adapter.
>
> fwiw, the valid use-case for ACPI boot on these things is for distro
> installer.. it might not be the shiny accelerated experience, but you
> want to be able to get thru the installer and then install updates to
> get latest kernel/dtb/etc
>
> it is a small use-case, but kinda an important step ;-)
>

That is a fair point. However, as I understand it, we need this to work around
- the need to pass efi=novamap
- broken poweroff on Flex5g

So an installer either needs to set the EFI variable, or pass
efi=novamap on the first boot. Note that there are no arm64 EFI
systems known where efi=novamap causes problems. In fact, I would
prefer to stop using SetVirtualAddressMap() altogether, as it does not
provide any benefit whatsoever. So perhaps we should make efi=novamap
the default and be done with it.

Broken poweroff is hardly a showstopper for an installer, given that
we cannot even install GRUB correctly.

In summary, I am more than happy to collaborate constructively on this
(which is why I wrote the patch), but I don't think we're at a point
yet where this is the only thing standing in our way when it comes to
a smooth out-of-the-box Linux installation experience.