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Date: Wed, 31 Jan 2018 23:26:50 -0500
From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To: KarimAllah Ahmed <karahmed@...zon.com>
Cc: Jim Mattson <jmattson@...gle.com>,
KarimAllah Ahmed <karahmed@...zon.de>,
kvm list <kvm@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
the arch/x86 maintainers <x86@...nel.org>,
Asit Mallick <asit.k.mallick@...el.com>,
Arjan Van De Ven <arjan.van.de.ven@...el.com>,
Dave Hansen <dave.hansen@...el.com>,
Andi Kleen <ak@...ux.intel.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Thomas Gleixner <tglx@...utronix.de>,
Dan Williams <dan.j.williams@...el.com>,
Jun Nakajima <jun.nakajima@...el.com>,
Paolo Bonzini <pbonzini@...hat.com>,
David Woodhouse <dwmw@...zon.co.uk>,
Greg KH <gregkh@...uxfoundation.org>,
Andy Lutomirski <luto@...nel.org>,
Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to
MSR_IA32_SPEC_CTRL
> >From 9c19a8ac3f021efba6f70ad7e28f7ad06bb97e43 Mon Sep 17 00:00:00 2001
> From: KarimAllah Ahmed <karahmed@...zon.de>
> Date: Mon, 29 Jan 2018 19:58:10 +0000
> Subject: [PATCH] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL
>
> [ Based on a patch from Ashok Raj <ashok.raj@...el.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 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.
^^^^^^^^^^^^^^^^^^^^^
That part of the comment does not seem to be in sync with the code.
>
> 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]
>
> Cc: Asit Mallick <asit.k.mallick@...el.com>
> Cc: Arjan Van De Ven <arjan.van.de.ven@...el.com>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Jun Nakajima <jun.nakajima@...el.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: David Woodhouse <dwmw@...zon.co.uk>
> Cc: Greg KH <gregkh@...uxfoundation.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Ashok Raj <ashok.raj@...el.com>
> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
> v6:
> - got rid of save_spec_ctrl_on_exit
> - introduce spec_ctrl_intercepted
> - introduce spec_ctrl_used
> v5:
> - Also check for X86_FEATURE_SPEC_CTRL for the msr reads/writes
> v4:
> - Add IBRS to kvm_cpuid_8000_0008_ebx_x86_features
> - Handling nested guests
> v3:
> - Save/restore manually
> - Fix CPUID handling
> - Fix a copy & paste error in the name of SPEC_CTRL MSR in
> disable_intercept.
> - support !cpu_has_vmx_msr_bitmap()
> 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 | 9 +++--
> arch/x86/kvm/vmx.c | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++-
> arch/x86/kvm/x86.c | 2 +-
> 3 files changed, 100 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 1909635..13f5d42 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -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;
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 6a9f4ec..bfc80ff 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -594,6 +594,14 @@ struct vcpu_vmx {
> #endif
>
> u64 arch_capabilities;
> + u64 spec_ctrl;
> +
> + /*
> + * This indicates that:
> + * 1) guest_cpuid_has(X86_FEATURE_IBRS) == true &&
> + * 2) The guest has actually initiated a write against the MSR.
> + */
> + bool spec_ctrl_used;
>
> /*
> * This indicates that:
> @@ -946,6 +954,8 @@ 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);
> @@ -1917,6 +1927,22 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
> vmcs_write32(EXCEPTION_BITMAP, eb);
> }
>
> +/* Is SPEC_CTRL intercepted for the currently running vCPU? */
> +static bool spec_ctrl_intercepted(struct kvm_vcpu *vcpu)
> +{
> + unsigned long *msr_bitmap;
> + int f = sizeof(unsigned long);
> +
> + if (!cpu_has_vmx_msr_bitmap())
> + return true;
> +
> + msr_bitmap = is_guest_mode(vcpu) ?
> + to_vmx(vcpu)->nested.vmcs02.msr_bitmap :
> + to_vmx(vcpu)->vmcs01.msr_bitmap;
> +
> + return !!test_bit(MSR_IA32_SPEC_CTRL, msr_bitmap + 0x800 / f);
> +}
> +
> static void clear_atomic_switch_msr_special(struct vcpu_vmx *vmx,
> unsigned long entry, unsigned long exit)
> {
> @@ -3246,6 +3272,14 @@ 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))
> @@ -3359,6 +3393,34 @@ 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;
> +
> + vmx->spec_ctrl_used = true;
> +
> + /* 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;
> +
> + /*
> + * When it's written (to non-zero) for the first time, pass
> + * it through. This means we don't have to take the perf
.. But only if it is a nested guest (as you have && is_guest_mode).
Do you want to update the comment a bit?
> + * hit of saving it on vmexit for the common case of guests
> + * that don't use it.
> + */
> + if (cpu_has_vmx_msr_bitmap() && data &&
> + spec_ctrl_intercepted(vcpu) &&
> + is_guest_mode(vcpu))
^^^^^^^^^^^^^^^^^^ <=== here
> + 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) &&
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