Re: [Xen-devel] linux-4.7.* under xen-4.6.* gives "unhandled page fault (ec=0000)" at boot. -- Root cause found.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.08.16 at 01:14, <hakon@xxxxxxxxxxxxxxxxxx> wrote:

First of all - please don't cross post.

> Den 17. aug. 2016 21:56, I wrote (to xen-users, as I am no developer):
>> I'm on gentoo, running gentoo-sources kernel for dom0.
>>
>> I am unable to run gentoo-sources-4.7.{0,1}. I'm running under xen,
>> currently at 4.6.3-r1
> 
> I am now on linux gentoo-sources-4.7.2, and the bug is stil present.
> 
> I have done some digging on my own, and found that protecting two instances 
> of " for_each_efi_memory_desc(md) {" with something like "if( efi.memmap.map 
> != NULL) {" will allow my dom0 to boot and run.

Which is basically the equivalent of e.g.

http://lkml.iu.edu/hypermail/linux/kernel/1608.2/03448.html

(this first one that a quick search found)

> My line-numbers may be a bit off due to copious use of pr_info.
> 
> The locations are:
> - linux/arch/x86/platform/efi/efi.c line 121 in function efi_find_mirror()
> - /usr/src/linux/arch/x86/platform/efi/quirks.c line 253 in function
> efi_free_boot_services(void)
> 
> Searching the web lead me first to a solution which I believe to be
> wrong, changing the comparison operator in for_each_efi_memory_desc. It
> also lead me to someone mentioning that efi.memmap.map is not used when
> running under xen. That info may also be bogus for all I know.

Not sure which part the "bogus" here is meant to apply to: Do you
mean the statement is bogus (in which case I wonder why), or do
you mean the data pointed to would be bogus (in which case I
wonder what data you think about when the pointer is NULL)?

Jan


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