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

From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
To: Shawn Guo <shawn.guo@linaro.org>
Cc: Ard Biesheuvel <ardb@kernel.org>,
	Heinrich Schuchardt <xypron.glpk@gmx.de>,
	Atish Patra <atish.patra@wdc.com>,
	Steve McIntyre <steve@einval.com>,
	Rob Clark <robdclark@gmail.com>,
	linux-efi <linux-efi@vger.kernel.org>,
	Jeffrey Hugo <jhugo@codeaurora.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Leif Lindholm <leif@nuviainc.com>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>
Subject: Re: [PATCH] efi: stub: override RT_PROP table supported mask based on EFI variable
Date: Tue, 16 Mar 2021 11:33:23 +0200	[thread overview]
Message-ID: <YFB7Y0FSSXmPEByF@enceladus> (raw)
In-Reply-To: <20210316090609.GB32651@dragon>

Hi Shawn,

> > > > > > 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.
> > > > >
> > > > > There might be more to be done for getting a smooth Linux installation
> > > > > experience.  But IMHO, this `OverrideSupported` thing is definitely
> > > > > a big step to that.
> > > > >
> > > >
> > > > So the problem here seems to be that grub-install (or efibootmgr)
> > > > tolerates efivarfs being absent entirely, but bails out if it exists
> > > > but gives an error when trying to access it, right?
> > >
> > > Yes, with EFI variables runtime service marked as unsupported,
> > > efibootmgr will just exit on efi_variables_supported() check [1] in
> > > a way that its parent process, i.e. grub-install, doesn't take as an
> > > error.  But otherwise, efibootmgr will go much further and exit with
> > > a real error when trying to access efivars.
> > >
> > 
> > OK, so I suggest we fix efibootmgr, by extending the
> > efi_variables_supported() check to perform a GetVariable() call on an
> > arbitrary variable (e.g., BootOrder),
> 
> Hmm, I'm not sure we should ask more from user space, as it's already
> been doing the right thing, and efi_variables_supported() is proved to
> work properly with any sane low-level software (kernel + firmware),
> either EFI variables service is supported or not.  That said, IMHO,
> right now the low-level software on Snapdragon laptops is insane, i.e.
> the unsupported/broken EFI runtime services are not communicated to
> user space properly in established way.

But the EFI_UNSUPPORTED is an error that's allowed from the spec. 
Yes the sane thing to do is not expose it at all, but it's not violating
any spec by doing so.
So why shouldn't a userspace application be able to handle all return codes
explicitly and instead treat them as a single error? And when that happens why
should the kernel mask that error out for it?

Thanks
/Ilias
> 
> Shawn
> 
> > and treat EFI_UNSUPPORTED as a
> > special case that means that carrying on is pointless.
> > 
> > (but someone please confirm that the snapdragon efi firmware does
> > return EFI_UNSUPPORTED and not some other return value when calling
> > GetVariable() from under the OS)