Re: Xen 4.18.5 PV dbregs fail

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 26/05/2025 7:46 pm, Manuel Bouyer wrote:
> On Mon, May 26, 2025 at 07:17:13PM +0100, Andrew Cooper wrote:
>> On 26/05/2025 7:06 pm, Andrew Cooper wrote:
>>> On 26/05/2025 6:59 pm, Manuel Bouyer wrote:
>>>> Hello,
>>>> since I updated to Xen 4.18.5 (from 4.18.4), NetBSD's dbregs-related tests
>>>> are failing. Only for PV; PVH and HVM guests are fine. They are
>>>> failing for both 32bits and 64bits guests.
>>>>
>>>> I tracked it down to dr6 being 0xffff0ff0 after the trace trap, when at
>>>> last one of the lower bits should be 1 (I think it's bit 0, from the code).
>>>>
>>>> I looked at the commit log between 4.18.4 and 4.18.5 but didn't see
>>>> anything obvious.
>>>>
>>>> Any idea ?
>>>>
>>> Have you got a link to the test in question?
>>>
>>> We've had a couple of bugfixes in this area, although debug handling
>>> isn't helped by the fact that both the SDM and APM are factually
>>> incorrect on how to write a #DB handler (and the vendors are moving at a
>>> glacial pace to correct them).
>> But yes, having looked between 4.18.4 and 4.18.5, I can't see anything
>> relevant to debug handling either.
>>
>> Judging from your description, it looks like a breakpoint is going missing.
>>
>> The relevant recent change for that is
>> https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=54ef601a66e8d812a6a6a308f02524e81201825e
>> although it's a bit older than 4.18.4.
> Well, my previous xen-debug.gz kernel is from Oct, 3 so it can't be
> 4.18.4, it's 4.18.3_20240909 which points to commit
> bd51e573a730efc569646379cd59ccba967cde97
>
> looks like I forgot to update this server. So I need to looks deeper in
> the commit logs

Well, that range does include the aforementioned commit.

Can you bisect around d32c77f471fb8400b6512c171a14cdd58f04f0a3 which is
the backport of ^ in 4.18 ?

~Andrew