[PATCH 00/10] arm64 kexec kernel patches V5

[Grant Likely]

On 7 Nov 2014 11:36, "Mark Rutland" <mark.rutland at arm.com> wrote:
>
> On Fri, Nov 07, 2014 at 10:41:11AM +0000, Ard Biesheuvel wrote:
> > On 7 November 2014 11:16, Mark Rutland <mark.rutland at arm.com> wrote:
> > > On Fri, Nov 07, 2014 at 12:41:45AM +0000, Grant Likely wrote:
> > >> On Thu, Nov 6, 2014 at 3:08 PM, Mark Rutland <mark.rutland at arm.com> wrote:
> > >> > On Thu, Nov 06, 2014 at 01:56:42AM +0000, Dave Young wrote:
> > >> >> On 11/03/14 at 07:46pm, Mark Rutland wrote:
> > >> >> > On Fri, Oct 31, 2014 at 07:52:09AM +0000, Dave Young wrote:
> > >> >> > > Hi Geoff
> > >> >> > >
> > >> >> > > I tested your patches. The macihne is using spin-table cpu enable method
> > >> >> > > so I tried maxcpus=1 as you suggested.
> > >> >> > >
> > >> >> > > There's below issues for me, thoughts?
> > >> >> > >
> > >> >> > > 1. For acpi booting there's no /proc/device-tree so kexec can not find dtb
> > >> >> > > to use.
> > >> >> >
> > >> >> > Are you absolutely certain of this?
> > >> >> >
> > >> >> > To use ACPI, you must have booted via EFI, as the only mechanism for
> > >> >> > finding the ACPI tables is via EFI. If booted via EFI, the stub will
> > >> >> > have created a stub DTB if there is no provided DTB, to pass the command
> > >> >> > line and pointers to EFI data structures. This stub DTB should be
> > >> >> > present in the usual place.
> > >> >>
> > >> >> Mark, I used kexec-tools from Geoff's git tree, it will create dtb from procfs
> > >> >> maybe I can pass external dtb to kexec-tools. What you mentioned should be true
> > >> >> though but I have not get idea how to get the dtb which kernel is using for boot
> > >> >> since it is not unflattened.
> > >> >
> > >> > Ah, sorry. I see the problem now. For ACPI you don't unflatten the tree,
> > >> > so there's nothing to expose at in sysfs/procfs.
> > >> >
> > >> > Somehow we need to unflatten the DTB without exposing it to drivers,
> > >> > such that it can be exposed to userspace in the usual place but drivers
> > >> > don't being probing based off of it.
> > >>
> > >> Is that even necessary? If the tree isn't unflattened, then it is just
> > >> a stub tree. There really isn't anything interesting in it.
> > >
> > > We need to UEFI properties [1] from /chosen to access the memory map and
> > > system table (both of which are necessary to access any ACPI tables).
> > >
> > >> Kexec should recreate the tree from scratch in that case.
> > >
> > > I'm not sure if the required information is exposed to userspace
> > > elsewhere. Ard, Leif?
> > >
> >
> > Personally, I think we should not be using /proc/device-tree at all,
> > but instead retain the original blob in some way and expose that.
>
> Grant took objection to that approach previously.

I don't strongly object, but using the original DTB is problematic if
the kernel makes changes to the tree that should be passed through to
the next kernel. The only big objection I have is to using the
existing debugfs implementation.

Care also need to be taken to make sure the flat tree has not been
modified on the fly. I think there are some scenarios where the kernel
modifies the DT buffer in-place.

That said, for a stub DT, the best place to generate it is still
probably in kernel space.

>
> > We already have /sys/firmware/efi/systab which contains the physical
> > addresses of the UEFI configuration tables. We should probably add an
> > entry for the FDT there anyway, but we would still be looking at
> > mmap(/dev/mem) to access it, which is not a practice we want to
> > encourage, I suppose.
>
> We should not encourage use of /dev/mem.
>
> Using /sys/firmware/efi/systab doesn't get us the memory map though,
> unless that's also exposed?
>
> > Also, we *must* take the secure boot scenario into account. Booting
> > with an arbitrary user generated DTB is nice, but if you are doing
> > kexec without an initrd, for instance, it would also be nice if we
> > could just reuse the existing DTB without bothering the user for it at
> > all, which would be something we could also allow when running
> > securely.
>
> Secure boot has to be handled completely differently. That will require
> new syscall support, in-kernel purgatory, and so on.
>
> That should not affect the non-secureboot cases where a user wants to
> load their own DTB (or other objects) into memory for the next OS (which
> might not be Linux).
>
> Mark.