Re: [PATCH] x86/PV: ignore PAE_MODE ELF note for 64-bit Dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.02.2023 19:41, Andrew Cooper wrote:
> On 15/02/2023 3:10 pm, Jan Beulich wrote:
>> Besides a printk() the main effect is slight corruption of the start
>> info magic: While that's meant to be xen-3.0-x86_64, it wrongly ended
>> up as xen-3.0-x86_64p.
>>
>> Fixes: 460060f83d41 ("libelf: use for x86 dom0 builder")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> RFC: While Linux works fine with the adjustment, I'm not entirely
>>      certain of external tools (kexec?) having grown a dependency.
> 
> Again, I wonder why you think regular kexec has any anything to do with
> this?
> 
> Are you mixing it up with (legacy) pvgrub which does end up doing a
> kexec inside the context of a PV guest?  If so, it's MiniOS's support
> you need to care about, and I cant see any logic that even inspects the
> start_info->magic[] (either in MiniOS itself, or the pvgrub patches).

I guess I really was thinking of something like "crash" instead, mixing
things up that way.

> Like plenty of other fields in the undocumented PV ABI, it's not
> interesting at all to software.  There's nothing I can see in it that
> you don't know at compile time.
> 
>>  It
>>      may be worth noting that XenoLinux and its forward ports never had
>>      this ELF note in 64-bit kernels, so in principle it may be
>>      reasonable to expect that no such dependency exists anywhere.
> 
> NetBSD sets PAE_MODE for 32bit and 64bit but, like everything else,
> doesn't appear to inspect start_info->magic[].

I.e. the same as (so far) upstream Linux.

> xen-3.0-x86_64p is obviously bogus.  I find it far more likely that
> noone has ever noticed this bug, than anyone gaining a dependency on it.

Which kind of reads like an ack, except there wasn't any ...

Jan