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Re: Xen 4.18.5 PV dbregs fail


  • To: Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 26 May 2025 21:30:53 +0100
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  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
  • Delivery-date: Mon, 26 May 2025 20:31:06 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 26/05/2025 9:13 pm, Manuel Bouyer wrote:
> On Mon, May 26, 2025 at 07:50:14PM +0100, Andrew Cooper wrote:
>> [...]
>> Well, that range does include the aforementioned commit.
>>
>> Can you bisect around d32c77f471fb8400b6512c171a14cdd58f04f0a3 which is
>> the backport of ^ in 4.18 ?
> Sure,
> with 0d5f15e and d32c77f the test pass, with cecee35 it fails.
>

Oh interesting, so the basic forwarding of #DB back into a guest
(d32c77f) works fine, but the changes to emulated debug exceptions
(cecee35) break.

Anyway, I think I've spotted a logical error.  We do indeed end up
calling x86_merge_dr6() twice, because of the TODO just out of context. 
Does this help?

~Andrew


diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 6d75b59b1e97..01b8be02b055 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1798,7 +1798,7 @@ void asmlinkage do_debug(struct cpu_user_regs *regs)
         return;
     }
 
-    pv_inject_DB(0 /* N/A, already merged */);
+    pv_inject_DB(dr6 ^ X86_DR6_DEFAULT);
 }
 
 void asmlinkage do_entry_CP(struct cpu_user_regs *regs)




 


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