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Re: [PATCH] x86: Add Kconfig option to use a 32bit TLB clock on debug
- To: Jan Beulich <jbeulich@xxxxxxxx>, Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 12 Jan 2026 14:47:52 +0000
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 12 Jan 2026 14:48:07 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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'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
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