Re: [PATCH v6 6/6] arm64: Use SYSTEM_OFF2 PSCI call to power off for hibernate

[Lorenzo Pieralisi <lpieralisi@kernel.org>]

On Mon, Nov 04, 2024 at 02:54:12PM +0100, Ard Biesheuvel wrote:
> On Fri, 1 Nov 2024 at 18:49, Lorenzo Pieralisi <lpieralisi@kernel.org> wrote:
> >
> > [+Ard, Sami, for EFI]
> >
> > On Thu, Oct 31, 2024 at 06:55:43PM +0100, Lorenzo Pieralisi wrote:
> > > On Sat, Oct 19, 2024 at 06:15:47PM +0100, David Woodhouse wrote:
> > >
> > > [...]
> > >
> > > > +#ifdef CONFIG_HIBERNATION
> > > > +static int psci_sys_hibernate(struct sys_off_data *data)
> > > > +{
> > > > +   /*
> > > > +    * Zero is an acceptable alternative to PSCI_1_3_OFF_TYPE_HIBERNATE_OFF
> > > > +    * and is supported by hypervisors implementing an earlier version
> > > > +    * of the pSCI v1.3 spec.
> > > > +    */
> > >
> > > It is obvious but with this patch applied a host kernel would start executing
> > > SYSTEM_OFF2 too if supported in firmware to hibernate, it is not a hypervisor
> > > only code path.
> > >
> > > Related to that: is it now always safe to override
> > >
> > > commit 60c0d45a7f7a ("efi/arm64: use UEFI for system reset and poweroff")
> > >
> > > for hibernation ? It is not very clear to me why overriding PSCI for
> > > poweroff was the right thing to do - tried to follow that patch history but
> > > the question remains (it is related to UpdateCapsule() but I don't know
> > > how that applies to the hibernation use case).
> >
> > RFC: It is unclear to me what happens in current mainline if we try to
> > hibernate with EFI runtime services enabled and a capsule update pending (we
> > issue EFI ResetSystem(EFI_RESET_SHUTDOWN,..) which might not be compatible
> > with the reset required by the pending capsule update request) what happens
> > in this case I don't know but at least the choice is all contained in
> > EFI firmware.
> >
> > Then if in the same scenario now we are switching to PSCI SYSTEM_OFF2 for the
> > hibernate reset I suspect that what happens to the in-flight capsule
> > update requests strictly depends on what "reset" PSCI SYSTEM_OFF2 will
> > end up doing ?
> >
> > I think this is just a corner case and it is unlikely it has been ever
> > tested (is it even possible ? Looking at EFI folks) - it would be good
> > to clarify it at least to make sure we understand this code path.
> >
> 
> I'm not aware of any OS that actually uses capsule update at runtime
> (both Windows and Linux queue up the capsule and call the
> UpdateCapsule() runtime service at boot time after a reboot).
> 
> So it is unlikely that this would break anything, and I'd actually be
> inclined to disable capsule update at runtime altogether.
> 
> I will also note that hibernation with EFI is flaky in general, given
> that EFI memory regions may move around

Thank you for chiming in, I think we are OK (I don't think this patch
will create more issues than the ones that are already there for hibernate
anyway) - the reasoning behind the change is in the commit logs.

Lorenzo