Re: xen-unstable + 4.1.0-rc1: Dom0 kernel stalls in early boot

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 04/27/2015 11:40 AM, Sander Eikelenboom wrote:
> Monday, April 27, 2015, 4:55:06 PM, you wrote:
>
>> Hi David / Konrad,
>> Today i tried upgrading my dom0 kernel to 4.1-rc1, but it stalls in early boot.
>> Xen console was still reponsive so i dumped some info with the debug keys.
>> Serial log is attached.
>> The kernel boots fine on baremetal and my previous test kernel that was pulled
>> and compiled on 20150423 in the mergewindow boots fine (unfortunately my scripts
>> don't add an git commit version to the kernel name yet.
>> (but the 20150423 version has it's own issue when used as a PV guest kernel
>> with pci passthrough, pci-front can't connect somehow, which i was trying
>> to get at .. when i stumbled upon this one :-) ).
>> --
>> Sander
> Hrmm shoot, that even didn't need a full bisection
> (but i looked over it before sending the mails), tested by reverting
> this one commit, after that it boots fine:
>
> 61f01dd941ba9e06d2bf05994450ecc3d61b6b8b
> "x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue"
>
> Question is .. why does it boot on baremetal, but not under Xen ?
> (any ideas before getting this to LKML and x86 maintainers  / Linus ?)

We shouldn't be loading __KERNEL_DS to stack segment in PV, which is 
what this patch does.

The bug that this commit is trying to fix is not applicable to PV guests 
in the first place (and even if it is it would have to be fixed in the 
hypervisor) so we essentially need to avoid setting 
X86_BUG_SYSRET_SS_ATTRS flag in CPU initialization code. I need to find 
a good place to clear it early enough.

-boris