At the weekend, Alex Ionescu reported on Twitter about an unexpected behavior of the PnP PCI driver from AMD for systems with Ryzen processors. The accusation of hidden performance tuning was quickly raised. As it turned out, this is probably not the case. AMD's approach remains questionable anyway.
PnP-PCI driver manipulates the CPU
As Ionescu found out, the PCI driver for Windows looks for 19 hashed processes (EXE files) that quickly turned out to be 19 games. If the driver detects one of the applications, it switches off certain functions via the model-specific register (MSR) of the CPU. With Ryzen 1000 (Zen 1, therefore probably also Ryzen 2000), certain functions in the cache for instructions (Op cache) are affected, with Ryzen 3000 (Zen 2) certain functions in the L1 and L2 cache are affected. On the other hand, the function does not seem to be available on Ryzen 5000.
Originally, Ionescu assumed that AMD would use functions that were advantageous for performance and that could lead to instability 19 applications activated – performance optimization is practically hidden via the PCI driver. In the meantime, however, he has corrected himself: The driver deactivates these functions, most likely to avoid stability problems.
The approach remains questionable
AMD's approach remains questionable: Neither the bit in the MSR that was adapted by the driver, nor the function in the driver is documented, according to Ionescu. The fact that the driver has also received the WHQL certificate from Microsoft, although it performs completely different tasks in addition to its actual function in the background, is also criticized. Although the PCI driver should even achieve the goal pursued with WHQL of increasing system stability, the hidden function does not create trust in the functionality of the driver.
Another one The problem concerns security: Ionescu was able to use the ioctl interface of the driver to crash the computer with just one line of code using PowerShell. In addition, it is possible to manipulate the list of processes monitored by the driver. AMD has not commented yet.
Crash a Ryzen system in single line of tweetable PowerShell:
(Get-NtFile Device NTPNP_PCI0031) .DeviceIoControl (0x9C402400, 5, 5)
Hey, AMD, If you're gonna twiddle magic MSR bits that control the instruction cache when playing certain video games, can you at least code properly https://t.co/hb0jB5bCxh pic.twitter.com/ryNzbQYSFL
& mdash; Alex Ionescu (@aionescu) May 16, 2021