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Need help with SMI, Intel Core i5-1135G7 CPU, and SMI induced latencies.




This is a repost of the following question, but to the Software category:


I believe that this is a hardware question but I'm hopeful to see if anyone in the Software category can help me instead. The above post (in Hardware Products) as well as a separate support ticket have both instructed me to repost this to the Software category on the Forums. It seems that the software group has quite the reputation for solving difficult problems :-).


Of the software sub-categories, it looks like "Software Tuning, Performance Optimization" is most applicable as my goal is to remove/reduce latencies caused by Dell laptops. The same Intel CPU (i5 1135G7) on another laptop has no latencies, which is why I suspect that this is caused by Dell's BIOS and/or hardware implementation.


Here's a copy of the original information/questions (slightly modified for clarity):


I have two laptops:

  • Lenovo ThinkBook Gen 2
  • Dell Latitude 5420

Both laptops have an Intel Core i5-1135G7 CPU.

I'm monitoring SMI (System Management Interrupts) by reading MSR register 0x34 (SMI Counter). I assume that this counter can only be incremented when there's an SMI.

The Lenovo laptop is not generating any SMI when running our real-time application. The Dell is generating an SMI roughly every 14.648 seconds. Both laptops have the same bits set in the SMI Control and Enable register (SMI_EN, offset 0x30):

  • 28: eSPI SMI Enable (ESPI_SMI_EN)
  • 13: TCO Enable (TCO_EN)
  • 1: End of SMI (EOS)
  • 0: Global SMI Enable (GBL_SMI_EN)

The latency caused by the SMI on the Dell is usually over 1 ms (i.e. the OS and all our application code pauses during this time).

As a side note, I can also see this latency when running the Intel BIOS Implementation Test Suite (BITS) SMI Latency test. On the Lenovo, there's zero SMI and the latency is <150 us so it passes the test. Based on other testing, the latency on the Lenovo appears to be about 7 us with this type of SMI Latency test. The same SMI Latency test fails on the Dell and has one SMI. The latency on the Dell is over 2 ms when running the BITS SMI Latency test. If I modify the BITS SMI Latency test to run for a shorter duration, for example, only 3 seconds instead of 15 seconds, then it's possible to run it on the Dell laptop between the SMIs. When this happens, then the test shows zero SMI and a latency of about 5 us.

Some preliminary questions are:

- Is the SMI Counter (MSR register 0x34) only incremented when there is an SMI?

- Is there a way to determine the cause of an SMI? Since only TCO and eSPI are enabled, it must be one of them but is there a way to determine which one?

- Is TCO SMI only used for Watch Dog Timers (WDT)? If it generates an SMI then should I expect the system to reboot in that case? I don't have any WDT drivers enabled that would be utilizing the TCO SMI. Should TCO SMIs be disabled by default unless some driver sets it up?

- The SMI seems to occur periodically every ~14.648 seconds. Could that give me any more clues about the cause?

- I've never used eSPI before but it looks like a bus. Is there a way to query/determine what's on the bus that might be causing these SMI?

- It looks like the TCO SMI is enabled and locked so that I cannot disable it. Is there a similar mechanism for eSPI that is preventing me from disabling the eSPI SMI via the SMI Control and Enable register (SMI_EN, offset 0x30)?

I guess that's it for now. Any other guidance/advice would be appreciated. I've created a case with Dell but it doesn't seem to be going anywhere (yet). Ideally, I would like to remove/prevent the periodic SMI on the Dell laptop.




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