A Linux 6.15 Performance Regression Hits Modern AMD CPUs
The intention of the bisected commit is to mitigate Speculative Return Stack Overflow (SRSO) across guest/host boundaries. Except that the bit is set if KVM virtualization is simply enabled for the kernel build and not necessarily used. Unfortunately, the KVM module is loaded on most Linux distribution vendor configurations and not always used, especially by consumers and other bare metal server use. So this ends up affecting workloads run simply on a common Linux kernel build (with KVM enabled) and making use of a newer AMD processor. Both AMD Ryzen and AMD EPYC processors depend upon the SRSO mitigation.
Reverting that patch atop Linux 6.15 Git (Linux 6.15-rc4) restores the lost performance back to the level observed with Linux 6.14 stable. Or booting with "modprobe.blacklist=kvm,kvm_amd" to block loading of the KVM kernel module also recovers from the performance hit of the mentioned patch.
I raised my regression report findings in advance of this article to AMD engineer Borislav Petkov who authored the patch that introduced the Linux 6.15 performance regression. Boris is going to be working on thinking of a solution to this issue for supporting SRSO_MSR_FIX with KVM but without introducing the performance penalty if KVM isn't even being utilized. As of writing though there isn't any solution available. So if you are running Linux 6.15 Git on a modern AMD system be aware of potentially hitting this issue if you are building KVM as part of your kernel configuration even if it's unused. Or you can revert 8442df2b49ed9bcd67833ad4f091d15ac91efd00 or blacklist the KVM modules to workaround this lost CPU performance currently on Linux 6.15 Git.
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