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Re: [PATCH v10] x86: use / "support" UDB
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 9 Apr 2026 07:50:45 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 09 Apr 2026 05:51:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 08.04.2026 20:04, Andrew Cooper wrote:
> 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>
Thanks.
>> ---
>> 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?
That's also okay, sure.
Jan
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