Re: [PATCH 06/12] ARM: kexec: advertise location of bootable RAM

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Mon, May 02, 2016 at 01:04:28PM +0530, Pratyush Anand wrote:
> On Sat, Apr 30, 2016 at 1:50 PM, Russell King - ARM Linux
> <linux@arm.linux.org.uk> wrote:
> > On Sat, Apr 30, 2016 at 08:57:34AM +0530, Pratyush Anand wrote:
> >> On Fri, Apr 29, 2016 at 11:30 PM, Russell King - ARM Linux
> >> <linux@arm.linux.org.uk> wrote:
> >> > On Fri, Apr 29, 2016 at 08:26:00PM +0530, Pratyush Anand wrote:
> >> >> Hi Russell,
> >> >>
> >> >> On Thu, Apr 28, 2016 at 2:58 PM, Russell King
> >> >> <rmk+kernel@arm.linux.org.uk> wrote:
> >> >> > Advertise the location of bootable RAM to kexec-tools.  kexec needs to
> >> >> > know where it can place the kernel in RAM, and so be executable when
> >> >> > the system needs to jump into it.
> >> >> >
> >> >> > Advertise these areas in /proc/iomem with a "System RAM (boot alias)"
> >> >> > tag.
> >> >> >
> >> >> > Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
> >> >>
> >> >> Can you please also share git tree path of corresponding kexec-tools changes?
> >> >>
> >> >> Could it be a better idea (if things in user space become simpler)
> >> >> that in stead of patch 5 and 6, we pass arch_phys_to_idmap_offset to
> >> >> user space, and then user space manipulates existing "Crash kernel"
> >> >> and "System RAM" resources.
> >> >
> >> > Given that it's only _one_ platform right now, I don't think that
> >> > additional complexity is worth it.  It means that we have to invent
> >>
> >> Probably, I could not communicate it well.  I was not trying  to have
> >> *additional* complexity. Wanted to see if things could be simpler
> >> rather. So this is what my understanding was:
> >> -- We create one patch to pass arch_phys_to_idmap_offset to user space
> >> (say in /sys/kernel/bootmem_idmap_offset)
> >> -- We do not use patch 5,6,11 and 12 of this series. Probably few more
> >> content of the series will go away.
> >
> > Patches 11 and 12 don't go away with what you're suggesting.  Patches
> > 11 and 12 are necessary to allow the boot-view addresses to be passed
> > into the kernel through kexec, and to allow kexec to find appropriate
> > memory resources.
> 
> But once we would have manipulated "start" and "end" of "Crash Kernel"
> and "System RAM" resources in user space using
> /sys/kernel/bootmem_idmap_offset , then kernel through kexec system
> call would have already receive boot-view addresses, no?

Correct, but that's still a problem for all the reasons I gave in the
email to which you replied to.

I'm not sure where the misunderstanding is.

Let me repeat: even if we do what you're suggesting, patches 11 and 12
do *not* go away.  I've explained in detail why each of the changes are
necessary (which you have cut from your reply.)

In other words: exporting this offset via
/sys/kernel/bootmem_idmap_offset is technically inferior to the solution
I have come up with here, and it saves very little complexity and code.

-- 
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