[PATCH 4.4 48/88] KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL

Greg Kroah-Hartman
2018-12-14T12:20:42Z

From: KarimAllah Ahmed <karahmed@amazon.de>

commit d28b387fb74da95d69d2615732f50cceb38e9a4d upstream.

[ Based on a patch from Ashok Raj <ashok.raj@intel.com> ]

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 saving and restoring the MSR_IA32_SPEC_CTRL for
guests that do not actually use the MSR, only start saving and restoring
when a non-zero is written to it.

No attempt is made to handle STIBP here, intentionally. Filtering STIBP
may be added in a future patch, which may require trapping all writes
if we don't want to pass it through directly to the guest.

[dwmw2: Clean up CPUID bits, save/restore manually, handle reset]

Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jun Nakajima <jun.nakajima@intel.com>
Cc: kvm@vger.kernel.org
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Asit Mallick <asit.k.mallick@intel.com>
Cc: Arjan Van De Ven <arjan.van.de.ven@intel.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ashok Raj <ashok.raj@intel.com>
Link: https://lkml.kernel.org/r/1517522386-18410-5-git-send-email-karahmed@amazon.de
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/cpuid.c | 8 ++-
arch/x86/kvm/cpuid.h | 11 +++++
arch/x86/kvm/vmx.c | 103 ++++++++++++++++++++++++++++++++++++++++++++++++++-
arch/x86/kvm/x86.c | 2
4 files changed, 118 insertions(+), 6 deletions(-)

--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -343,7 +343,7 @@ static inline int __do_cpuid_ent(struct

/* cpuid 0x80000008.ebx */
const u32 kvm_cpuid_8000_0008_ebx_x86_features =
- F(IBPB);
+ F(IBPB) | F(IBRS);

/* cpuid 0xC0000001.edx */
const u32 kvm_supported_word5_x86_features =
@@ -364,7 +364,7 @@ static inline int __do_cpuid_ent(struct

/* cpuid 7.0.edx*/
const u32 kvm_cpuid_7_0_edx_x86_features =
- F(ARCH_CAPABILITIES);
+ F(SPEC_CTRL) | F(ARCH_CAPABILITIES);

/* all calls to cpuid_count() should be made on the same cpu */
get_cpu();
@@ -595,9 +595,11 @@ static inline int __do_cpuid_ent(struct
g_phys_as = phys_as;
entry->eax = g_phys_as | (virt_as << 8);
entry->edx = 0;
- /* IBPB isn't necessarily present in hardware cpuid */
+ /* IBRS and IBPB aren't necessarily present in hardware cpuid */
if (boot_cpu_has(X86_FEATURE_IBPB))
entry->ebx |= F(IBPB);
+ if (boot_cpu_has(X86_FEATURE_IBRS))
+ entry->ebx |= F(IBRS);
entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
break;
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -170,6 +170,17 @@ static inline bool guest_cpuid_has_ibpb(
return best && (best->edx & bit(X86_FEATURE_SPEC_CTRL));
}

+static inline bool guest_cpuid_has_ibrs(struct kvm_vcpu *vcpu)
+{
+ struct kvm_cpuid_entry2 *best;
+
+ best = kvm_find_cpuid_entry(vcpu, 0x80000008, 0);
+ if (best && (best->ebx & bit(X86_FEATURE_IBRS)))
+ return true;
+ best = kvm_find_cpuid_entry(vcpu, 7, 0);
+ return best && (best->edx & bit(X86_FEATURE_SPEC_CTRL));
+}
+
static inline bool guest_cpuid_has_arch_capabilities(struct kvm_vcpu *vcpu)
{
struct kvm_cpuid_entry2 *best;
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -546,6 +546,7 @@ struct vcpu_vmx {
#endif

u64 arch_capabilities;
+ u64 spec_ctrl;

u32 vm_entry_controls_shadow;
u32 vm_exit_controls_shadow;
@@ -1693,6 +1694,29 @@ static void update_exception_bitmap(stru
}

/*
+ * Check if MSR is intercepted for currently loaded MSR bitmap.
+ */
+static bool msr_write_intercepted(struct kvm_vcpu *vcpu, u32 msr)
+{
+ unsigned long *msr_bitmap;
+ int f = sizeof(unsigned long);
+
+ if (!cpu_has_vmx_msr_bitmap())
+ return true;
+
+ msr_bitmap = to_vmx(vcpu)->loaded_vmcs->msr_bitmap;
+
+ if (msr <= 0x1fff) {
+ return !!test_bit(msr, msr_bitmap + 0x800 / f);
+ } else if ((msr >= 0xc0000000) && (msr <= 0xc0001fff)) {
+ msr &= 0x1fff;
+ return !!test_bit(msr, msr_bitmap + 0xc00 / f);
+ }
+
+ return true;
+}
+
+/*
* Check if MSR is intercepted for L01 MSR bitmap.
*/
static bool msr_write_intercepted_l01(struct kvm_vcpu *vcpu, u32 msr)
@@ -2834,6 +2858,13 @@ static int vmx_get_msr(struct kvm_vcpu *
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_ibrs(vcpu))
+ return 1;
+
+ msr_info->data = to_vmx(vcpu)->spec_ctrl;
+ break;
case MSR_IA32_ARCH_CAPABILITIES:
if (!msr_info->host_initiated &&
!guest_cpuid_has_arch_capabilities(vcpu))
@@ -2939,6 +2970,36 @@ static int vmx_set_msr(struct kvm_vcpu *
case MSR_IA32_TSC:
kvm_write_tsc(vcpu, msr_info);
break;
+ case MSR_IA32_SPEC_CTRL:
+ if (!msr_info->host_initiated &&
+ !guest_cpuid_has_ibrs(vcpu))
+ return 1;
+
+ /* The STIBP bit doesn't fault even if it's not advertised */
+ if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP))
+ return 1;
+
+ vmx->spec_ctrl = data;
+
+ if (!data)
+ break;
+
+ /*
+ * For non-nested:
+ * When it's written (to non-zero) for the first time, pass
+ * it through.
+ *
+ * For nested:
+ * The handling of the MSR bitmap for L2 guests is done in
+ * nested_vmx_merge_msr_bitmap. We should not touch the
+ * vmcs02.msr_bitmap here since it gets completely overwritten
+ * in the merging. We update the vmcs01 here for L1 as well
+ * since it will end up touching the MSR anyway now.
+ */
+ vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap,
+ MSR_IA32_SPEC_CTRL,
+ MSR_TYPE_RW);
+ break;
case MSR_IA32_PRED_CMD:
if (!msr_info->host_initiated &&
!guest_cpuid_has_ibpb(vcpu))
@@ -5045,6 +5106,7 @@ static void vmx_vcpu_reset(struct kvm_vc
u64 cr0;

vmx->rmode.vm86_active = 0;
+ vmx->spec_ctrl = 0;

vmx->soft_vnmi_blocked = 0;

@@ -8589,6 +8651,15 @@ static void __noclone vmx_vcpu_run(struc
atomic_switch_perf_msrs(vmx);
debugctlmsr = get_debugctlmsr();

+ /*
+ * If this vCPU has touched SPEC_CTRL, restore the guest's value if
+ * it's non-zero. Since vmentry is serialising on affected CPUs, there
+ * is no need to worry about the conditional branch over the wrmsr
+ * being speculatively taken.
+ */
+ if (vmx->spec_ctrl)
+ wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
+
vmx->__launched = vmx->loaded_vmcs->launched;
asm(
/* Store host registers */
@@ -8707,6 +8778,27 @@ static void __noclone vmx_vcpu_run(struc
#endif
);

+ /*
+ * We do not use IBRS in the kernel. If this vCPU has used the
+ * SPEC_CTRL MSR it may have left it on; save the value and
+ * turn it off. This is much more efficient than blindly adding
+ * it to the atomic save/restore list. Especially as the former
+ * (Saving guest MSRs on vmexit) doesn't even exist in KVM.
+ *
+ * For non-nested case:
+ * If the L01 MSR bitmap does not intercept the MSR, then we need to
+ * save it.
+ *
+ * For nested case:
+ * If the L02 MSR bitmap does not intercept the MSR, then we need to
+ * save it.
+ */
+ if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))
+ rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
+
+ if (vmx->spec_ctrl)
+ wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+
/* Eliminate branch target predictions from guest mode */
vmexit_fill_RSB();

@@ -9242,7 +9334,7 @@ static inline bool nested_vmx_merge_msr_
unsigned long *msr_bitmap_l1;
unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.vmcs02.msr_bitmap;
/*
- * pred_cmd is trying to verify two things:
+ * pred_cmd & spec_ctrl are trying to verify two things:
*
* 1. L0 gave a permission to L1 to actually passthrough the MSR. This
* ensures that we do not accidentally generate an L02 MSR bitmap
@@ -9255,9 +9347,10 @@ static inline bool nested_vmx_merge_msr_
* the MSR.
*/
bool pred_cmd = msr_write_intercepted_l01(vcpu, MSR_IA32_PRED_CMD);
+ bool spec_ctrl = msr_write_intercepted_l01(vcpu, MSR_IA32_SPEC_CTRL);

if (!nested_cpu_has_virt_x2apic_mode(vmcs12) &&
- !pred_cmd)
+ !pred_cmd && !spec_ctrl)
return false;

page = nested_get_page(vcpu, vmcs12->msr_bitmap);
@@ -9293,6 +9386,12 @@ static inline bool nested_vmx_merge_msr_
}
}

+ if (spec_ctrl)
+ nested_vmx_disable_intercept_for_msr(
+ msr_bitmap_l1, msr_bitmap_l0,
+ MSR_IA32_SPEC_CTRL,
+ MSR_TYPE_R | MSR_TYPE_W);
+
if (pred_cmd)
nested_vmx_disable_intercept_for_msr(
msr_bitmap_l1, msr_bitmap_l0,
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -961,7 +961,7 @@ 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_ARCH_CAPABILITIES
+ MSR_IA32_SPEC_CTRL, MSR_IA32_ARCH_CAPABILITIES
};

static unsigned num_msrs_to_save;