rep movsb's prefetch logic

[wanghw]

When I used the CPU mentioned above, I discovered that executing memcpy with rep movsb accesses addresses beyond the range of send_buf, resulting in an exception warning message.When I adjust the threshold of glibc.cpu.x86_rep_movsb_threshold to be greater than the length of send_buf, no exception is triggered.Thus, I speculate that rep movsb accessed an address outside of send_buf.Therefore, I would like to inquire about the optimizations made for the rep movsb instruction in this CPU.

cpu info：https://www.intel.com/content/www/us/en/products/sku/215269/intel-xeon-silver-4314-processor-24m-cache-2-40-ghz/specifications.html

OS：Red Hat Enterprise Linux release 9.1 (Plow)

glibc：(GNU libc) 2.34

[RandyT_Intel]

Hi @wanghw,

I will forward your inquiry to our respective team at Xeon so that you can receive further assistance on this specific concern. Please anticipate a response from one of our team members shortly

Regards,

Randy T.

Intel Customer Support Technician

[wanghw]

After disabling Fast-Strings Enable, I did not encounter any abnormal warning messages. The Software Developer’s Manual mentions in section 7.3.9 that when fast-string operation is enabled (because IA32_MISC_ENABLE[0] = 1), some processors may further enhance the operation of the REP MOVSB and REP STOSB instructions. Could you please explain how this enhancement occurs?

[Sachinks]

Hello wanghw,

Thank you for posting in Intel community Forum.

Based on your query, kindly provide us the below details so that we can understand the issue better and assist you further.

1) Exact exception or warning message you are seeing

2) Steps/methods you are following to allocate the threshold

3) Kernel version and if there was any recent microcode updates done for the CPU

4) Also, kindly provide us the server/motherboard model details in which you are using this CPU

Regards,

Sachin KS

Intel Customer Support Technician

[Sachinks]

Hello wanghw,

Thank you for your reply. 

We are glad to know that after disabling Fast-Strings Enable, you did not encounter any abnormal warning messages.

Regarding your query related to Fast-Strings Enable, we request you post your query in our developer zone page by creating a developer zone premier account.

Link to our developer zone page : https://www.intel.com/content/www/us/en/developer/overview.html#gs.m4re67

How to Apply for an Intel® Resource and Documentation Center (RDC) and/or Intel® Developer Zone premier (Intel® DevZone) Account:

https://www.intel.com/content/www/us/en/support/articles/000058073/programs/resource-and-documentation-center.html

Additionally, please note that standard Corporate Non-Disclosure Agreement (CNDA) is required to create the developer zone premier account. 

If you do not have an active CNDA, we recommend contacting your Intel® Representative / Distributors from your organization, who will be able to assist you in getting the CNDA signed with Intel.

Regards,

Sachin KS

Intel Customer Support Technician

[wanghw]

Alright, thank you, I will give it a try.

[Sachinks]

Hello wanghw,

Thank you for your understanding. We will close this thread for now.

Please don't hesitate to ask any further questions in the future. Feel free to start a new conversation, as this thread will no longer be monitored.

Regards,

Sachin KS

Intel Customer Support Technician