Re: [PATCH] x86: Add Kconfig option to use a 32bit TLB clock on debug

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.01.2026 16:28, Alejandro Vallejo wrote:
> On Mon Jan 12, 2026 at 3:47 PM CET, Andrew Cooper wrote:
>> On 12/01/2026 2:43 pm, Jan Beulich wrote:
>>> On 12.01.2026 15:08, Alejandro Vallejo wrote:
>>>> Debug builds stress the wrapping logic of the TLB clock by narrowing it
>>>> down to 10 bits. This is inconvenient to test real time workloads on
>>>> such builds.
>>>>
>>>> Add Kconfig option to be able to selectively use the non-stressed
>>>> behaviour on debug.
>>>>
>>>> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
>>> Hmm, yes, why not. However, ...
>>>
>>>> --- a/xen/arch/x86/flushtlb.c
>>>> +++ b/xen/arch/x86/flushtlb.c
>>>> @@ -20,11 +20,7 @@
>>>>  #include <asm/spec_ctrl.h>
>>>>  
>>>>  /* Debug builds: Wrap frequently to stress-test the wrap logic. */
>>>> -#ifdef NDEBUG
>>>> -#define WRAP_MASK (0xFFFFFFFFU)
>>>> -#else
>>>> -#define WRAP_MASK (0x000003FFU)
>>>> -#endif
>>>> +#define WRAP_MASK (IS_ENABLED(CONFIG_DEBUG_TLB_CLK) ? 0x3FFU : UINT32_MAX)
>>> ... the comment then will want updating as well, I'd say. It doesn't go
>>> terribly stale this way, but at least slightly. I'd suggest to minimally
>>> drop "builds".
> 
> I left the comment because the rationale still holds. Dropping "builds" sounds
> good to me.
> 
>>
>> I'm suggest just dropping WRAP_MASK.
>>
>> We've done this locally in the XenServer patchqueue since 2011 or so due
>> to the overhead, and I don't think it's interesting enough to warrant a
>> separate option.
>>
>> ~Andrew
> 
> I don't mind much either way. I need it gone for my needs and I don't care 
> much
> how it happens.
> 
> Jan + Roger, do you have strong opinions on the matter?

Dropping altogether is fine with me.

Jan