Re: dom0 PV looping on search_pre_exception_table()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.12.2020 10:51, Manuel Bouyer wrote:
> On Wed, Dec 09, 2020 at 07:08:41PM +0000, Andrew Cooper wrote:
>> Oh of course - we don't follow the exit-to-guest path on the way out here.
>>
>> As a gross hack to check that we've at least diagnosed the issue
>> appropriately, could you modify NetBSD to explicitly load the %ss
>> selector into %es (or any other free segment) before first entering user
>> context?
> 
> If I understood it properly, the user %ss is loaded by Xen from the
> trapframe when the guest swictes from kernel to user mode, isn't it ?
> So you mean setting %es to the same value in the trapframe ?
> 
> Actually I used %fs because %es is set equal to %ds.
> Xen 4.13 boots fine with this change, but with 4.15 I get a loop of:
> 
> 
> (XEN) *** LDT: gl1e 0000000000000000 not present                              
>  
> (XEN) *** pv_map_ldt_shadow_page(0x40) failed                                 
>  

Could you please revert 9ff970564764 ("x86/mm: drop guest_get_eff_l1e()")?
I think there was a thinko there in that the change can't be split from
the bigger one which was part of the originally planned set for XSA-286.
We mustn't avoid the switching of page tables as long as
guest_get_eff{,_kern}_l1e() makes use of the linear page tables.

Jan