Re: [PATCH]kdump: pass noefi and acpi_rsdp= to 2nd kernel

[Dave Young <dyoung@redhat.com>]

On 09/07/2012 01:57 AM, Khalid Aziz wrote:

> On 09/05/2012 09:18 PM, Dave Young wrote:
>>
>>> I think running a kexec/kdump kernel with "noefi" is not a good idea.
>>> Today kernel makes very little use of UEFI run time services but this
>>> might change shortly. I have already seen ideas being proposed to use
>>> UEFI variables to store kernel panic information which would require
>>> accessing UEFI runtime services. If the kdump kernel runs into a panic,
>>> it would be good to be able to use UEFI variables to store some sort of
>>> tombstone. We might also start using UEFI runtime clock services one of
>>> these days especially now that all PCs soon will have UEFI due to
>>> Windows 8 requirements. There might be other ways to deal with EFI
>>> virtualization issue. I have solved it on a custom ia64 kernel by
>>> passing the kexec'd kernel a "kexec_reboot" on command line, although I
>>> wouldn't recommend that approach as a general approach. Creating a new
>>> kernel command line parameter just to tell the kernel to not virtualize
>>> EFI sounds excessive. May be we can come up with a different way to tell
>>> a kexec/kdump kernel to not virtualize EFI, like create a new signature,
>>> for example "EL32_KEXEC" and "EL64_KEXEC", for
>>> boot_params.efi_info.efi_loader_signature which tells the kexec/kdump
>>> kernel to enable EFI but skip the step of virtualizing it. More work
>>> will be needed to make this work, for example pass the EFI runtime
>>> service memory map from one kernel to the next so we keep it mapped in
>>> exact same spot, and other similar mapping issues.
>>>
>>
>> I'm not so familiar with uefi detail, is the virtualization issue mean
>> "virtual mode" of efi?  From my understanding for kdump kernel I would
>> vote for not interacting with bios at all. We have use "noefi" for long
>> time by passing it to 2nd kernel while kexecing. I prefer to do this way
>> until we have to switch to other approach.
>>
> Yes, that is what I mean by virtualizing issue. Using "noefi" for kdump
> kernel will keep it from using any runtime EFI services. That could mean
> kdump kernel may not be able to read clock or use EFI variables to store
> a tombstone in case it runs into trouble. Is that acceptable?
> 
> For the kexec'd kernel (not kdump kernel), being able to use runtime EFI
> services will be essential as we move to EFI machines. We will have to
> support enabling EFI on a kexec'd kernel. Does that sound reasonable?
> 


It's probably needed, but currently relevant implementation  is not
ready especially efi init code for kdump. We can address this once it's ok.

I did some digging about the efi boot kdump in slackware with kernel
from latest linus tree. I'm surprise that kdump works even without these
"noefi" and "acpi_rsdp="

With fedora 17 3.4.3 kernel passing "acpi_rsdp=" is enough for kdump
kernel booting.

Actually in x86 setup code if bootloader do not pass correct efi_info in
boot_params efi_enabled will be set to 0 automaticlly.

So as for this patch 'noefi' is not needed.

I will do more testing to see why linus tree works without "acpi_rsdp="


-- 
Thanks
Dave

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