Re: [PATCH v2 2/4] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL
From: KarimAllah Ahmed
Date: Mon Jan 29 2018 - 07:16:48 EST
On 01/29/2018 11:44 AM, Paolo Bonzini wrote:
On 29/01/2018 01:58, KarimAllah Ahmed wrote:
Add direct access to MSR_IA32_SPEC_CTRL for guests. This is needed for
guests that will only mitigate Spectre V2 through IBRS+IBPB and will not
be using a retpoline+IBPB based approach.
To avoid the overhead of atomically saving and restoring the
MSR_IA32_SPEC_CTRL for guests that do not actually use the MSR, only
add_atomic_switch_msr when a non-zero is written to it.
You are not storing the guest's MSR value on though vmexit, aren't you?
I originally thought that atomic_switch was also saving the guest MSR on
VM-exit. Now I know it is not.
Also, there's an obvious typo here:
+ add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL, msr_info->data, 0);
+
+ msr_bitmap = vmx->vmcs01.msr_bitmap;
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_FS_BASE, MSR_TYPE_RW);
+
oops! copy & paste error :)
Finally, apparently add_atomic_switch_msr is slower than just rdmsr/wrmsr
on vmexit. Can you reuse the patches I had posted mid January instead? They
are also assuming no IBRS usage on the host, so the changes shouldn't be large,
and limited mostly to using actual X86_FEATURE_* bits instead of cpuid_count().
They lack the code to only read/write SPEC_CTRL if the direct access is enabled,
but that's small too... Enabling the direct access on the first write, as in
this patches, is okay.
Thanks,
Paolo
Cc: Asit Mallick <asit.k.mallick@xxxxxxxxx>
Cc: Arjan Van De Ven <arjan.van.de.ven@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Ashok Raj <ashok.raj@xxxxxxxxx>
Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
---
v2:
- remove 'host_spec_ctrl' in favor of only a comment (dwmw@).
- special case writing '0' in SPEC_CTRL to avoid confusing live-migration
when the instance never used the MSR (dwmw@).
- depend on X86_FEATURE_IBRS instead of X86_FEATURE_SPEC_CTRL (dwmw@).
- add MSR_IA32_SPEC_CTRL to the list of MSRs to save (dropped it by accident).
---
arch/x86/kvm/cpuid.c | 4 +++-
arch/x86/kvm/vmx.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++
arch/x86/kvm/x86.c | 1 +
3 files changed, 69 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 0099e10..32c0c14 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -70,6 +70,7 @@ u64 kvm_supported_xcr0(void)
/* These are scattered features in cpufeatures.h. */
#define KVM_CPUID_BIT_AVX512_4VNNIW 2
#define KVM_CPUID_BIT_AVX512_4FMAPS 3
+#define KVM_CPUID_BIT_IBRS 26
#define KF(x) bit(KVM_CPUID_BIT_##x)
int kvm_update_cpuid(struct kvm_vcpu *vcpu)
@@ -392,7 +393,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
/* cpuid 7.0.edx*/
const u32 kvm_cpuid_7_0_edx_x86_features =
- KF(AVX512_4VNNIW) | KF(AVX512_4FMAPS);
+ KF(AVX512_4VNNIW) | KF(AVX512_4FMAPS) | \
+ (boot_cpu_has(X86_FEATURE_IBRS) ? KF(IBRS) : 0);
/* all calls to cpuid_count() should be made on the same cpu */
get_cpu();
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index aa8638a..dac564d 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -920,6 +920,8 @@ static void vmx_set_nmi_mask(struct kvm_vcpu *vcpu, bool masked);
static bool nested_vmx_is_page_fault_vmexit(struct vmcs12 *vmcs12,
u16 error_code);
static void vmx_update_msr_bitmap(struct kvm_vcpu *vcpu);
+static void __always_inline vmx_disable_intercept_for_msr(unsigned long *msr_bitmap,
+ u32 msr, int type);
static DEFINE_PER_CPU(struct vmcs *, vmxarea);
static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
@@ -2007,6 +2009,28 @@ static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
m->host[i].value = host_val;
}
+/* do not touch guest_val and host_val if the msr is not found */
+static int read_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
+ u64 *guest_val, u64 *host_val)
+{
+ unsigned i;
+ struct msr_autoload *m = &vmx->msr_autoload;
+
+ for (i = 0; i < m->nr; ++i)
+ if (m->guest[i].index == msr)
+ break;
+
+ if (i == m->nr)
+ return 1;
+
+ if (guest_val)
+ *guest_val = m->guest[i].value;
+ if (host_val)
+ *host_val = m->host[i].value;
+
+ return 0;
+}
+
static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
{
u64 guest_efer = vmx->vcpu.arch.efer;
@@ -3203,7 +3227,9 @@ static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
*/
static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
{
+ u64 spec_ctrl = 0;
struct shared_msr_entry *msr;
+ struct vcpu_vmx *vmx = to_vmx(vcpu);
switch (msr_info->index) {
#ifdef CONFIG_X86_64
@@ -3223,6 +3249,20 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
case MSR_IA32_TSC:
msr_info->data = guest_read_tsc(vcpu);
break;
+ case MSR_IA32_SPEC_CTRL:
+ if (!msr_info->host_initiated &&
+ !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
+ return 1;
+
+ /*
+ * If the MSR is not in the atomic list yet, then the guest
+ * never wrote a non-zero value to it yet i.e. the MSR value is
+ * '0'.
+ */
+ read_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL, &spec_ctrl, NULL);
+
+ msr_info->data = spec_ctrl;
+ break;
case MSR_IA32_SYSENTER_CS:
msr_info->data = vmcs_read32(GUEST_SYSENTER_CS);
break;
@@ -3289,6 +3329,7 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
int ret = 0;
u32 msr_index = msr_info->index;
u64 data = msr_info->data;
+ unsigned long *msr_bitmap;
switch (msr_index) {
case MSR_EFER:
@@ -3330,6 +3371,30 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
case MSR_IA32_TSC:
kvm_write_tsc(vcpu, msr_info);
break;
+ case MSR_IA32_SPEC_CTRL:
+ if (!msr_info->host_initiated &&
+ !guest_cpuid_has(vcpu, X86_FEATURE_IBRS))
+ return 1;
+
+ if (!msr_info->data)
+ break;
+
+ /*
+ * Now we know that the guest is actually using the MSR, so
+ * atomically load and save the SPEC_CTRL MSR and pass it
+ * through to the guest.
+ *
+ * NOTE:
+ * IBRS is not supported yet as a mitigation for the host. Once
+ * it is supported, the "host_value" will need to be '1'
+ * instead of '0' if IBRS is used also by the host.
+ */
+ add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL, msr_info->data, 0);
+
+ msr_bitmap = vmx->vmcs01.msr_bitmap;
+ vmx_disable_intercept_for_msr(msr_bitmap, MSR_FS_BASE, MSR_TYPE_RW);
+
+ break;
case MSR_IA32_CR_PAT:
if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 03869eb..cabaad3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1006,6 +1006,7 @@ static u32 msrs_to_save[] = {
#endif
MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
MSR_IA32_FEATURE_CONTROL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
+ MSR_IA32_SPEC_CTRL
};
static unsigned num_msrs_to_save;
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