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KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL

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[ 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
This commit is contained in:
KarimAllah Ahmed 2018-02-01 22:59:45 +01:00 committed by Thomas Gleixner
parent 28c1c9fabf
commit d28b387fb7
3 changed files with 110 additions and 6 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

9
arch/x86/kvm/cpuid.c
View file

@ -367,7 +367,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
/* cpuid 0x80000008.ebx */
const u32 kvm_cpuid_8000_0008_ebx_x86_features =
F(IBPB);
F(IBPB) | F(IBRS);
/* cpuid 0xC0000001.edx */
const u32 kvm_cpuid_C000_0001_edx_x86_features =
@ -394,7 +394,8 @@ 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 =
F(AVX512_4VNNIW) | F(AVX512_4FMAPS) | F(ARCH_CAPABILITIES);
F(AVX512_4VNNIW) | F(AVX512_4FMAPS) | F(SPEC_CTRL) |
F(ARCH_CAPABILITIES);
/* all calls to cpuid_count() should be made on the same cpu */
get_cpu();
@ -630,9 +631,11 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
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;

105
arch/x86/kvm/vmx.c
View file

@ -595,6 +595,7 @@ struct vcpu_vmx {
#endif
u64 arch_capabilities;
u64 spec_ctrl;
u32 vm_entry_controls_shadow;
u32 vm_exit_controls_shadow;
@ -1910,6 +1911,29 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
vmcs_write32(EXCEPTION_BITMAP, eb);
}
/*
* 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.
*/
@ -3262,6 +3286,14 @@ 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_IBRS) &&
!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
return 1;
msr_info->data = to_vmx(vcpu)->spec_ctrl;
break;
case MSR_IA32_ARCH_CAPABILITIES:
if (!msr_info->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_ARCH_CAPABILITIES))
@ -3375,6 +3407,37 @@ 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) &&
!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
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(vcpu, X86_FEATURE_IBPB) &&
@ -5700,6 +5763,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
u64 cr0;
vmx->rmode.vm86_active = 0;
vmx->spec_ctrl = 0;
vmx->vcpu.arch.regs[VCPU_REGS_RDX] = get_rdx_init_val();
kvm_set_cr8(vcpu, 0);
@ -9371,6 +9435,15 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
vmx_arm_hv_timer(vcpu);
/*
* 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 */
@ -9489,6 +9562,27 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
#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();
@ -10113,7 +10207,7 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
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
@ -10126,9 +10220,10 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
* 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 = kvm_vcpu_gpa_to_page(vcpu, vmcs12->msr_bitmap);
@ -10162,6 +10257,12 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
}
}
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,

2
arch/x86/kvm/x86.c
View file

@ -1009,7 +1009,7 @@ 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;