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commite3417ab75aupstream. Set the magic BP_SPEC_REDUCE bit to mitigate SRSO when running VMs if and only if KVM has at least one active VM. Leaving the bit set at all times unfortunately degrades performance by a wee bit more than expected. Use a dedicated spinlock and counter instead of hooking virtualization enablement, as changing the behavior of kvm.enable_virt_at_load based on SRSO_BP_SPEC_REDUCE is painful, and has its own drawbacks, e.g. could result in performance issues for flows that are sensitive to VM creation latency. Defer setting BP_SPEC_REDUCE until VMRUN is imminent to avoid impacting performance on CPUs that aren't running VMs, e.g. if a setup is using housekeeping CPUs. Setting BP_SPEC_REDUCE in task context, i.e. without blasting IPIs to all CPUs, also helps avoid serializing 1<=>N transitions without incurring a gross amount of complexity (see the Link for details on how ugly coordinating via IPIs gets). Link: https://lore.kernel.org/all/aBOnzNCngyS_pQIW@google.com Fixes:8442df2b49("x86/bugs: KVM: Add support for SRSO_MSR_FIX") Reported-by: Michael Larabel <Michael@michaellarabel.com> Closes: https://www.phoronix.com/review/linux-615-amd-regression Cc: Borislav Petkov <bp@alien8.de> Tested-by: Borislav Petkov (AMD) <bp@alien8.de> Link: https://lore.kernel.org/r/20250505180300.973137-1-seanjc@google.com Signed-off-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>