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Message-Id: <1517669783-20732-1-git-send-email-karahmed@amazon.de>
Date: Sat, 3 Feb 2018 15:56:23 +0100
From: KarimAllah Ahmed <karahmed@...zon.de>
To: linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org
Cc: KarimAllah Ahmed <karahmed@...zon.de>,
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: [PATCH v7] KVM: SVM: Allow direct access to MSR_IA32_SPEC_CTRL
[ Based on a patch from Paolo Bonzini <pbonzini@...hat.com> ]
... basically doing exactly what we do for VMX:
- Passthrough SPEC_CTRL to guests (if enabled in guest CPUID)
- Save and restore SPEC_CTRL around VMExit and VMEntry only if the guest
actually used it.
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:
- ditch save_spec_ctrl_on_exit in favor of msr_write_intercepted
v5:
- Add SPEC_CTRL to direct_access_msrs.
---
arch/x86/kvm/svm.c | 88 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 88 insertions(+)
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 254eefb..4e3c795 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -184,6 +184,8 @@ struct vcpu_svm {
u64 gs_base;
} host;
+ u64 spec_ctrl;
+
u32 *msrpm;
ulong nmi_iret_rip;
@@ -249,6 +251,7 @@ static const struct svm_direct_access_msrs {
{ .index = MSR_CSTAR, .always = true },
{ .index = MSR_SYSCALL_MASK, .always = true },
#endif
+ { .index = MSR_IA32_SPEC_CTRL, .always = false },
{ .index = MSR_IA32_PRED_CMD, .always = false },
{ .index = MSR_IA32_LASTBRANCHFROMIP, .always = false },
{ .index = MSR_IA32_LASTBRANCHTOIP, .always = false },
@@ -882,6 +885,25 @@ static bool valid_msr_intercept(u32 index)
return false;
}
+static bool msr_write_intercepted(struct kvm_vcpu *vcpu, unsigned msr)
+{
+ u8 bit_write;
+ unsigned long tmp;
+ u32 offset;
+ u32 *msrpm;
+
+ msrpm = is_guest_mode(vcpu) ? to_svm(vcpu)->nested.msrpm:
+ to_svm(vcpu)->msrpm;
+
+ offset = svm_msrpm_offset(msr);
+ bit_write = 2 * (msr & 0x0f) + 1;
+ tmp = msrpm[offset];
+
+ BUG_ON(offset == MSR_INVALID);
+
+ return !!test_bit(bit_write, &tmp);
+}
+
static void set_msr_interception(u32 *msrpm, unsigned msr,
int read, int write)
{
@@ -1584,6 +1606,8 @@ static void svm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
u32 dummy;
u32 eax = 1;
+ svm->spec_ctrl = 0;
+
if (!init_event) {
svm->vcpu.arch.apic_base = APIC_DEFAULT_PHYS_BASE |
MSR_IA32_APICBASE_ENABLE;
@@ -3605,6 +3629,13 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
case MSR_VM_CR:
msr_info->data = svm->nested.vm_cr_msr;
break;
+ case MSR_IA32_SPEC_CTRL:
+ if (!msr_info->host_initiated &&
+ !guest_cpuid_has(vcpu, X86_FEATURE_IBRS))
+ return 1;
+
+ msr_info->data = svm->spec_ctrl;
+ break;
case MSR_IA32_UCODE_REV:
msr_info->data = 0x01000065;
break;
@@ -3696,6 +3727,33 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
case MSR_IA32_TSC:
kvm_write_tsc(vcpu, msr);
break;
+ case MSR_IA32_SPEC_CTRL:
+ if (!msr->host_initiated &&
+ !guest_cpuid_has(vcpu, X86_FEATURE_IBRS))
+ return 1;
+
+ /* The STIBP bit doesn't fault even if it's not advertised */
+ if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP))
+ return 1;
+
+ svm->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_svm_vmrun_msrpm.
+ * We update the L1 MSR bit as well since it will end up
+ * touching the MSR anyway now.
+ */
+ set_msr_interception(svm->msrpm, MSR_IA32_SPEC_CTRL, 1, 1);
+ break;
case MSR_IA32_PRED_CMD:
if (!msr->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_IBPB))
@@ -4964,6 +5022,15 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
local_irq_enable();
+ /*
+ * 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 (svm->spec_ctrl)
+ wrmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
+
asm volatile (
"push %%" _ASM_BP "; \n\t"
"mov %c[rbx](%[svm]), %%" _ASM_BX " \n\t"
@@ -5056,6 +5123,27 @@ static void svm_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, svm->spec_ctrl);
+
+ if (svm->spec_ctrl)
+ wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+
/* Eliminate branch target predictions from guest mode */
vmexit_fill_RSB();
--
2.7.4
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