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Re: [PATCH v10] x86: use / "support" UDB
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 8 Apr 2026 19:04:46 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>
- Delivery-date: Wed, 08 Apr 2026 18:05:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 08/04/2026 1:12 pm, Jan Beulich wrote:
> With opcode D6h now firmly reserved as another #UD-raising one in 64-bit
> mode, use that instead of the two-byte UD2 for bug frame marking.
>
> While there also make a minor adjustment to the emulator.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
For the patch itself, Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> One of the table entries in stub_selftest() uses UD1, yet in not quite
> an appropriate way: The 0x90 following it (presumably meant to be a NOP)
> really is a ModR/M byte, requiring a displacement to follow. Wouldn't we
> better adjust that (e.g. using 0xcc instead)?
That looks too much like breakpoint padding as opposed to nop padding.
What about:
diff --git a/xen/arch/x86/extable.c b/xen/arch/x86/extable.c
index a9b6c6b904f5..24440ccd03e3 100644
--- a/xen/arch/x86/extable.c
+++ b/xen/arch/x86/extable.c
@@ -157,7 +157,7 @@ int __init cf_check stub_selftest(void)
union stub_exception_token res;
} tests[] __initconst = {
#define endbr64 0xf3, 0x0f, 0x1e, 0xfa
- { .opc = { endbr64, 0x0f, 0xb9, 0x90 }, /* ud1 */
+ { .opc = { endbr64, 0x0f, 0xb9, 0x00 }, /* ud1 (%rax),%eax */
.res.fields.trapnr = X86_EXC_UD },
{ .opc = { endbr64, 0x90, 0x02, 0x00 }, /* nop; add (%rax),%al */
.rax = 0x0123456789abcdef,
which also brings it in line with the adjacent example?
~Andrew
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