Re: VMX Preemption Timer appears to be buggy on SKX, CLX, and ICX

[Chao Gao]

On Fri, Jun 12, 2026 at 03:51:56PM -0700, Jim Mattson wrote:
>On Thu, Jun 4, 2026 at 10:56 PM Chao Gao <chao.gao@xxxxxxxxx> wrote:
>>
>> On Thu, Jun 04, 2026 at 10:34:40PM -0700, Jim Mattson wrote:
>...
>> >With respect to EMR, you wrote previously, "A mitigation for this
>> >erratum is for software to program the VMX preemption timer for values
>> >below 2^25 * CPUID.15H:EBX[31:0] / CPUID.15H:EAX[31:0]."
>> >
>> >My question is whether the exponent, 25, is a fixed value for all
>> >CPUs, regardless of their IA32_VMX_MISC[4:0]. It sounds like you are
>> >saying that the exponent may depend on IA32_VMX_MISC[4:0].
>>
>> Let me double-check this with the internal team and get back to you.
>
>Have you discovered anything?

I just got the final answer:

2^25*M/N is the max value of VMX preemption timer for _all_ affected
processors.  Our expectation is that for processors that have different
value it will have the issue fixed (VMX preemption timer will implement
full 32-bit). Use of 2^25*M/N will still be fine.

where M is CPUID.15H:EBX[31:0] and N is CPUID.15H:EAX[31:0].