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

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

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?

Cheers,
Alejandro