KVM: VMX: Tell the nested hypervisor to skip L1D flush on vmentry

commit 5b76a3cff011df2dcb6186c965a2e4d809a05ad4 upstream

When nested virtualization is in use, VMENTER operations from the nested
hypervisor into the nested guest will always be processed by the bare metal
hypervisor, and KVM's "conditional cache flushes" mode in particular does a
flush on nested vmentry.  Therefore, include the "skip L1D flush on
vmentry" bit in KVM's suggested ARCH_CAPABILITIES setting.

Add the relevant Documentation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/l1tf.rst b/Documentation/admin-guide/l1tf.rst
index 5dadb4503ec991860e70efb878869fb9957c71af..bae52b845de0b93af644ea55103d5a912dfca753 100644 (file)
--- a/Documentation/admin-guide/l1tf.rst
+++ b/Documentation/admin-guide/l1tf.rst
@@ -546,6 +546,27 @@ available:
     EPT can be disabled in the hypervisor via the 'kvm-intel.ept'
     parameter.
 
+3.4. Nested virtual machines
+""""""""""""""""""""""""""""
+
+When nested virtualization is in use, three operating systems are involved:
+the bare metal hypervisor, the nested hypervisor and the nested virtual
+machine.  VMENTER operations from the nested hypervisor into the nested
+guest will always be processed by the bare metal hypervisor. If KVM is the
+bare metal hypervisor it wiil:
+
+ - Flush the L1D cache on every switch from the nested hypervisor to the
+   nested virtual machine, so that the nested hypervisor's secrets are not
+   exposed to the nested virtual machine;
+
+ - Flush the L1D cache on every switch from the nested virtual machine to
+   the nested hypervisor; this is a complex operation, and flushing the L1D
+   cache avoids that the bare metal hypervisor's secrets are exposed to the
+   nested virtual machine;
+
+ - Instruct the nested hypervisor to not perform any L1D cache flush. This
+   is an optimization to avoid double L1D flushing.
+
 
 .. _default_mitigations:
 

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index d6ff1b132b4749ee2e0d665344fecebacdbbb9f1..4015b88383ce98ad5cbc828f098bb0d3b8b16598 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1374,6 +1374,7 @@ int kvm_cpu_get_interrupt(struct kvm_vcpu *v);
 void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event);
 void kvm_vcpu_reload_apic_access_page(struct kvm_vcpu *vcpu);
 
+u64 kvm_get_arch_capabilities(void);
 void kvm_define_shared_msr(unsigned index, u32 msr);
 int kvm_set_shared_msr(unsigned index, u64 val, u64 mask);
 

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 63ec03108b5de0bfab3072d360e54ae12c0f3ce9..f015ca3997d9216c28c1db88b1e7e7cf0e6de64f 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -5910,8 +5910,7 @@ static int vmx_vcpu_setup(struct vcpu_vmx *vmx)
                ++vmx->nmsrs;
        }
 
-       if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
-               rdmsrl(MSR_IA32_ARCH_CAPABILITIES, vmx->arch_capabilities);
+       vmx->arch_capabilities = kvm_get_arch_capabilities();
 
        vm_exit_controls_init(vmx, vmcs_config.vmexit_ctrl);
 

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fcc058edfabaf9fca49aa1c26c292aeaaec4065b..5c2c09f6c1c31310ad0e8ab9c124bb29540fb377 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1054,11 +1054,35 @@ static u32 msr_based_features[] = {
 
 static unsigned int num_msr_based_features;
 
+u64 kvm_get_arch_capabilities(void)
+{
+       u64 data;
+
+       rdmsrl_safe(MSR_IA32_ARCH_CAPABILITIES, &data);
+
+       /*
+        * If we're doing cache flushes (either "always" or "cond")
+        * we will do one whenever the guest does a vmlaunch/vmresume.
+        * If an outer hypervisor is doing the cache flush for us
+        * (VMENTER_L1D_FLUSH_NESTED_VM), we can safely pass that
+        * capability to the guest too, and if EPT is disabled we're not
+        * vulnerable.  Overall, only VMENTER_L1D_FLUSH_NEVER will
+        * require a nested hypervisor to do a flush of its own.
+        */
+       if (l1tf_vmx_mitigation != VMENTER_L1D_FLUSH_NEVER)
+               data |= ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;
+
+       return data;
+}
+EXPORT_SYMBOL_GPL(kvm_get_arch_capabilities);
+
 static int kvm_get_msr_feature(struct kvm_msr_entry *msr)
 {
        switch (msr->index) {
-       case MSR_IA32_UCODE_REV:
        case MSR_IA32_ARCH_CAPABILITIES:
+               msr->data = kvm_get_arch_capabilities();
+               break;
+       case MSR_IA32_UCODE_REV:
                rdmsrl_safe(msr->index, &msr->data);
                break;
        default: