kdump/kexec on EFI-enabled x2apic platforms

Jack Steiner steiner at sgi.com
Mon Mar 29 16:01:04 EDT 2010

All -

I just started debugging kdump/kexec on our UV platform and
have run into some problems. I suspect others have encountered these
same or similar problems. Any help would be appreciated.

Our platform uses EFI boot. It is Nehalem based & has a large number of cpus.
The BIOS enables x2apic mode and the kernel runs with interrupt remapping enabled.
Note that some apicids have more than 8 bits - x2apic mode is required.

I am able to successfully kexec the dump kernel but run into several problems.

	- because the initial kernel boots using EFI, BIOS does not build the legacy
	  tables that are required to locate the RSDP using the legacy method in
	  acpi_find_root_pointer(). (When booting with EFI, acpi_find_root_pointer() is
	  not used. The ACPI tables are found from pointers in EFI tables.)

	- it appears that kdump/kexec intentionally boots the kdump kernel
	  in a mode does does enable efi mode. (Am I correct here???)
	  This avoids the issues with EFI virtual mode. However, the result
	  is that ACPI tables are not found. From the dump kernel:

	  	ACPI Error: A valid RSDP was not found (20090903/tbxfroot-222)

	- Because ACPI tables are not found, the dump kernel does not transition
	  into x2apic mode. The hardware, however, is still in x2apic mode from the
	  initial kernel. Later in the boot of the dump kernel, read_apic_id()
	  tried to read memory-mapped apic registers instead of the MSRs that are
	  used in x2apic mode. This is not allowed & the dump kernel panics with:

		[    0.000000]  [<ffffffff81b52195>] early_idt_handler+0x55/0x68
		[    0.000000]  [<ffffffff8101f393>] ? native_apic_mem_read+0x3/0x10
		[    0.000000]  [<ffffffff8101a3c6>] ? read_apic_id+0x16/0x30
		[    0.000000]  [<ffffffff81b5f857>] init_apic_mappings+0xe7/0x137
		[    0.000000]  [<ffffffff81b559fd>] setup_arch+0x900/0xc33
		[    0.000000]  [<ffffffff81b52bae>] start_kernel+0x6f/0x4a1

I checked an Intel Nehalem whitebox using the Intel BIOS. The dump kernel does not
find the RDSP but the initial kernel does not enable x2apic mode either (possibly
because of an old BIOS - not sure).  As a result, the dump kernel does not hit
the panic shown above. The kdump kernel successfully boots w/o having discovered
ACPI tables.

How should I proceed?
	- should I be running the dump kernel with EFI mode enabled?
	- should I be fixing the issues with x2apic mode in a non-EFI dump kernel?
	- or should BIOS be building the tables necessary to support both EFI & non-EFI boot.

--- jack

Jack Steiner (steiner at sgi.com)		SGI - Silicon Graphics, Inc.

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