Re: [PATCH 0/3] arm64: kexec,kdump: fix boot failures on acpi-only system

[AKASHI Takahiro]

James,

Thank you for follow-up explanation.
I have nothing to add :)

-Takahiro AKASHI

On Fri, Jun 15, 2018 at 05:29:32PM +0100, James Morse wrote:
> Hi Akashi,
> 
> Thanks for putting this together,
> 
> On 15/06/18 08:56, AKASHI Takahiro wrote:
> > This patch series is a set of bug fixes to address kexec/kdump
> > failures which are sometimes observed on ACPI-only system and reported
> > in LAK-ML before.
> > 
> > In short, the phenomena are:
> > 1. kexec'ed kernel can fail to boot because some ACPI table is corrupted
> >    by a new kernel (or other data) being loaded into System RAM. Currently
> >    kexec may possibly allocate space ignoring such "reserved" regions.
> >    We will see no messages after "Bye!"
> > 
> > 2. crash dump (kdump) kernel can fail to boot and get into panic due to
> >    an alignment fault when accessing ACPI tables. This can happen because
> >    those tables are not always properly aligned while they are mapped
> >    non-cacheable (ioremap'ed) as they are not recognized as part of System
> >    RAM under the current implementation.
> > 
> > After discussing several possibilities to address those issues,
> > the agreed approach, in my understanding, is
> > * to add resource entries for every "reserved", i.e. memblock_reserve(),
> >   regions to /proc/iomem.
> >   (NOMAP regions, also marked as "reserved," remains at top-level for
> >   backward compatibility.)
> 
> This means user-space can tell the difference between reserved-system-ram and
> reserved-address-space.
> 
> 
> > * For case (1), user space (kexec-tools) should rule out such regions
> >   in searching for free space for loaded data.
> 
> ... but doesn't today, because it fails to account for second-level entries.
> We've always had second-level entries, so this is a user-space bug. We need both
> fixed to fix the issue.
> 
> Our attempts to fix this just in the kernel reached a dead end, because Kdump
> needs to include reserved-system-ram, whereas kexec has to avoid it. User-space
> needs to be able to tell reserved-system-ram and reserved-address-space apart.
> Hence we need to expose that information, and pick it up in user-space.
> 
> Patched-kernel and unpatch-user-space will work the same way it does today, as
> the additional reserved regions are ignored by user-space.
> 
> Unpatched-kernel and patched-user-space will also work the same way it does
> today as the additional reserved regions are missing.
> 
> I think this is the only way forwards on this issue...
> 
> 
> > * For case (2), the kernel should access ACPI tables by mapping
> >   them with appropriate memory attributes described in UEFI memory map.
> >   (This means that it doesn't require any changes in /proc/iomem, and
> >   hence user space.)
> 
> (this one is handled entirely in the kernel)
> 
> 
> Thanks,
> 
> James