From: Sean Christopherson sean.j.christopherson@intel.com
commit bf09fb6cba4f7099620cc9ed32d94c27c4af992e upstream.
Remove support for context switching between the guest's and host's desired UMWAIT_CONTROL. Propagating the guest's value to hardware isn't required for correct functionality, e.g. KVM intercepts reads and writes to the MSR, and the latency effects of the settings controlled by the MSR are not architecturally visible.
As a general rule, KVM should not allow the guest to control power management settings unless explicitly enabled by userspace, e.g. see KVM_CAP_X86_DISABLE_EXITS. E.g. Intel's SDM explicitly states that C0.2 can improve the performance of SMT siblings. A devious guest could disable C0.2 so as to improve the performance of their workloads at the detriment to workloads running in the host or on other VMs.
Wholesale removal of UMWAIT_CONTROL context switching also fixes a race condition where updates from the host may cause KVM to enter the guest with the incorrect value. Because updates are are propagated to all CPUs via IPI (SMP function callback), the value in hardware may be stale with respect to the cached value and KVM could enter the guest with the wrong value in hardware. As above, the guest can't observe the bad value, but it's a weird and confusing wart in the implementation.
Removal also fixes the unnecessary usage of VMX's atomic load/store MSR lists. Using the lists is only necessary for MSRs that are required for correct functionality immediately upon VM-Enter/VM-Exit, e.g. EFER on old hardware, or for MSRs that need to-the-uop precision, e.g. perf related MSRs. For UMWAIT_CONTROL, the effects are only visible in the kernel via TPAUSE/delay(), and KVM doesn't do any form of delay in vcpu_vmx_run(). Using the atomic lists is undesirable as they are more expensive than direct RDMSR/WRMSR.
Furthermore, even if giving the guest control of the MSR is legitimate, e.g. in pass-through scenarios, it's not clear that the benefits would outweigh the overhead. E.g. saving and restoring an MSR across a VMX roundtrip costs ~250 cycles, and if the guest diverged from the host that cost would be paid on every run of the guest. In other words, if there is a legitimate use case then it should be enabled by a new per-VM capability.
Note, KVM still needs to emulate MSR_IA32_UMWAIT_CONTROL so that it can correctly expose other WAITPKG features to the guest, e.g. TPAUSE, UMWAIT and UMONITOR.
Fixes: 6e3ba4abcea56 ("KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL") Cc: stable@vger.kernel.org Cc: Jingqi Liu jingqi.liu@intel.com Cc: Tao Xu tao3.xu@intel.com Signed-off-by: Sean Christopherson sean.j.christopherson@intel.com Message-Id: 20200623005135.10414-1-sean.j.christopherson@intel.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/include/asm/mwait.h | 2 -- arch/x86/kernel/cpu/umwait.c | 6 ------ arch/x86/kvm/vmx/vmx.c | 18 ------------------ 3 files changed, 26 deletions(-)
diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h index b809f117f3f46..9d5252c9685c6 100644 --- a/arch/x86/include/asm/mwait.h +++ b/arch/x86/include/asm/mwait.h @@ -23,8 +23,6 @@ #define MWAITX_MAX_LOOPS ((u32)-1) #define MWAITX_DISABLE_CSTATES 0xf0
-u32 get_umwait_control_msr(void); - static inline void __monitor(const void *eax, unsigned long ecx, unsigned long edx) { diff --git a/arch/x86/kernel/cpu/umwait.c b/arch/x86/kernel/cpu/umwait.c index 300e3fd5ade3e..ec8064c0ae035 100644 --- a/arch/x86/kernel/cpu/umwait.c +++ b/arch/x86/kernel/cpu/umwait.c @@ -18,12 +18,6 @@ */ static u32 umwait_control_cached = UMWAIT_CTRL_VAL(100000, UMWAIT_C02_ENABLE);
-u32 get_umwait_control_msr(void) -{ - return umwait_control_cached; -} -EXPORT_SYMBOL_GPL(get_umwait_control_msr); - /* * Cache the original IA32_UMWAIT_CONTROL MSR value which is configured by * hardware or BIOS before kernel boot. diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 2d7ce012b1d5f..390ec34e4b4f1 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -6467,23 +6467,6 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx) msrs[i].host, false); }
-static void atomic_switch_umwait_control_msr(struct vcpu_vmx *vmx) -{ - u32 host_umwait_control; - - if (!vmx_has_waitpkg(vmx)) - return; - - host_umwait_control = get_umwait_control_msr(); - - if (vmx->msr_ia32_umwait_control != host_umwait_control) - add_atomic_switch_msr(vmx, MSR_IA32_UMWAIT_CONTROL, - vmx->msr_ia32_umwait_control, - host_umwait_control, false); - else - clear_atomic_switch_msr(vmx, MSR_IA32_UMWAIT_CONTROL); -} - static void vmx_update_hv_timer(struct kvm_vcpu *vcpu) { struct vcpu_vmx *vmx = to_vmx(vcpu); @@ -6576,7 +6559,6 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) pt_guest_enter(vmx);
atomic_switch_perf_msrs(vmx); - atomic_switch_umwait_control_msr(vmx);
if (enable_preemption_timer) vmx_update_hv_timer(vcpu);