kvm: x86: Require const tsc for RT

Non constant TSC is a nightmare on bare metal already, but with
virtualization it becomes a complete disaster because the workarounds
are horrible latency wise. That's also a preliminary for running RT in
a guest on top of a RT host.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Message-Id: <Yh5eJSG19S2sjZfy@linutronix.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4fa4d82..c58265d 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8865,6 +8865,12 @@ int kvm_arch_init(void *opaque)
                goto out;
        }
 
+       if (IS_ENABLED(CONFIG_PREEMPT_RT) && !boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
+               pr_err("RT requires X86_FEATURE_CONSTANT_TSC\n");
+               r = -EOPNOTSUPP;
+               goto out;
+       }
+
        r = -ENOMEM;
 
        x86_emulator_cache = kvm_alloc_emulator_cache();