Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in the
vCPU load path, introduce a static branch, similar to switch_mm_*_ibpb.
This makes it obvious in spectre_v2_user_select_mitigation() what
exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB
(which will be shortly removed). It also provides more fine-grained
control, making it simpler to change/add paths that control the IBPB in
the vCPU load path without affecting other IBPBs.
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
arch/x86/include/asm/nospec-branch.h | 2 ++
arch/x86/kernel/cpu/bugs.c | 5 +++++
arch/x86/kvm/svm/svm.c | 2 +-
arch/x86/kvm/vmx/vmx.c | 2 +-
4 files changed, 9 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 7cbb76a2434b9..a22836c5fb338 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
+DECLARE_STATIC_KEY_FALSE(vcpu_load_ibpb);
+
DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index a5d0998d76049..685a6f97fea8f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -113,6 +113,10 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
/* Control unconditional IBPB in switch_mm() */
DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
+/* Control IBPB on vCPU load */
+DEFINE_STATIC_KEY_FALSE(vcpu_load_ibpb);
+EXPORT_SYMBOL_GPL(vcpu_load_ibpb);
+
/* Control MDS CPU buffer clear before idling (halt, mwait) */
DEFINE_STATIC_KEY_FALSE(mds_idle_clear);
EXPORT_SYMBOL_GPL(mds_idle_clear);
@@ -1365,6 +1369,7 @@ spectre_v2_user_select_mitigation(void)
/* Initialize Indirect Branch Prediction Barrier */
if (boot_cpu_has(X86_FEATURE_IBPB)) {
setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
+ static_branch_enable(&vcpu_load_ibpb);
spectre_v2_user_ibpb = mode;
switch (cmd) {
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index a4ba5b4e3d682..043d56d276ad6 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1560,7 +1560,7 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
sd->current_vmcb = svm->vmcb;
if (!cpu_feature_enabled(X86_FEATURE_IBPB_ON_VMEXIT) &&
- cpu_feature_enabled(X86_FEATURE_USE_IBPB))
+ static_branch_likely(&vcpu_load_ibpb))
indirect_branch_prediction_barrier();
}
if (kvm_vcpu_apicv_active(vcpu))
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 729a8ee24037b..7f950d0b50757 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1478,7 +1478,7 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
* may switch the active VMCS multiple times).
*/
if (!buddy || WARN_ON_ONCE(buddy->vmcs != prev))
- if (cpu_feature_enabled(X86_FEATURE_USE_IBPB))
+ if (static_branch_likely(&vcpu_load_ibpb))
indirect_branch_prediction_barrier();
}
--
2.48.1.601.g30ceb7b040-googOn Wed, Feb 19, 2025, Yosry Ahmed wrote: > Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in the > vCPU load path, introduce a static branch, similar to switch_mm_*_ibpb. > > This makes it obvious in spectre_v2_user_select_mitigation() what > exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB > (which will be shortly removed). It also provides more fine-grained > control, making it simpler to change/add paths that control the IBPB in > the vCPU load path without affecting other IBPBs. > > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev> > --- > arch/x86/include/asm/nospec-branch.h | 2 ++ > arch/x86/kernel/cpu/bugs.c | 5 +++++ > arch/x86/kvm/svm/svm.c | 2 +- > arch/x86/kvm/vmx/vmx.c | 2 +- > 4 files changed, 9 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > index 7cbb76a2434b9..a22836c5fb338 100644 > --- a/arch/x86/include/asm/nospec-branch.h > +++ b/arch/x86/include/asm/nospec-branch.h > @@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp); > DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb); > DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb); > > +DECLARE_STATIC_KEY_FALSE(vcpu_load_ibpb); How about ibpb_on_vcpu_load? To make it easy for readers to understand exactly what the knob controls.
February 25, 2025 at 11:49 AM, "Sean Christopherson" <seanjc@google.com> wrote: > > On Wed, Feb 19, 2025, Yosry Ahmed wrote: > > > > Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in the > > vCPU load path, introduce a static branch, similar to switch_mm_*_ibpb. > > > > This makes it obvious in spectre_v2_user_select_mitigation() what > > exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB > > (which will be shortly removed). It also provides more fine-grained > > control, making it simpler to change/add paths that control the IBPB in > > the vCPU load path without affecting other IBPBs. > > > > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev> > > > > --- > > > > arch/x86/include/asm/nospec-branch.h | 2 ++ > > arch/x86/kernel/cpu/bugs.c | 5 +++++ > > arch/x86/kvm/svm/svm.c | 2 +- > > arch/x86/kvm/vmx/vmx.c | 2 +- > > 4 files changed, 9 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > > index 7cbb76a2434b9..a22836c5fb338 100644 > > --- a/arch/x86/include/asm/nospec-branch.h > > +++ b/arch/x86/include/asm/nospec-branch.h > > @@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp); > > DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb); > > DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb); > > +DECLARE_STATIC_KEY_FALSE(vcpu_load_ibpb); > > > > How about ibpb_on_vcpu_load? To make it easy for readers to understand exactly > what the knob controls. I was trying to remain consistent with the existing static branches' names, but I am fine with ibpb_on_vcpu_load if others don't object.
On Tue, Feb 25, 2025, Yosry Ahmed wrote: > February 25, 2025 at 11:49 AM, "Sean Christopherson" <seanjc@google.com> wrote: > > > > On Wed, Feb 19, 2025, Yosry Ahmed wrote: > > > > > > Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in the > > > vCPU load path, introduce a static branch, similar to switch_mm_*_ibpb. > > > > > > This makes it obvious in spectre_v2_user_select_mitigation() what > > > exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB > > > (which will be shortly removed). It also provides more fine-grained > > > control, making it simpler to change/add paths that control the IBPB in > > > the vCPU load path without affecting other IBPBs. > > > > > > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev> > > > > > > --- > > > > > > arch/x86/include/asm/nospec-branch.h | 2 ++ > > > arch/x86/kernel/cpu/bugs.c | 5 +++++ > > > arch/x86/kvm/svm/svm.c | 2 +- > > > arch/x86/kvm/vmx/vmx.c | 2 +- > > > 4 files changed, 9 insertions(+), 2 deletions(-) > > > > > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > > > index 7cbb76a2434b9..a22836c5fb338 100644 > > > --- a/arch/x86/include/asm/nospec-branch.h > > > +++ b/arch/x86/include/asm/nospec-branch.h > > > @@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp); > > > DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb); > > > > DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb); > > > > +DECLARE_STATIC_KEY_FALSE(vcpu_load_ibpb); > > > > > > > How about ibpb_on_vcpu_load? To make it easy for readers to understand exactly > > what the knob controls. > > I was trying to remain consistent with the existing static branches' names, > but I am fine with ibpb_on_vcpu_load if others don't object. I assumed as much :-) I'm ok with vcpu_load_ibpb if that's what others prefer.
On Tue, Feb 25, 2025 at 02:40:24PM -0800, Sean Christopherson wrote: > On Tue, Feb 25, 2025, Yosry Ahmed wrote: > > February 25, 2025 at 11:49 AM, "Sean Christopherson" <seanjc@google.com> wrote: > > > > > > On Wed, Feb 19, 2025, Yosry Ahmed wrote: > > > > > > > > Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in the > > > > vCPU load path, introduce a static branch, similar to switch_mm_*_ibpb. > > > > > > > > This makes it obvious in spectre_v2_user_select_mitigation() what > > > > exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB > > > > (which will be shortly removed). It also provides more fine-grained > > > > control, making it simpler to change/add paths that control the IBPB in > > > > the vCPU load path without affecting other IBPBs. > > > > > > > > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev> > > > > > > > > --- > > > > > > > > arch/x86/include/asm/nospec-branch.h | 2 ++ > > > > arch/x86/kernel/cpu/bugs.c | 5 +++++ > > > > arch/x86/kvm/svm/svm.c | 2 +- > > > > arch/x86/kvm/vmx/vmx.c | 2 +- > > > > 4 files changed, 9 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > > > > index 7cbb76a2434b9..a22836c5fb338 100644 > > > > --- a/arch/x86/include/asm/nospec-branch.h > > > > +++ b/arch/x86/include/asm/nospec-branch.h > > > > @@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp); > > > > DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb); > > > > > > DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb); > > > > > > +DECLARE_STATIC_KEY_FALSE(vcpu_load_ibpb); > > > > > > > > > > How about ibpb_on_vcpu_load? To make it easy for readers to understand exactly > > > what the knob controls. > > > > I was trying to remain consistent with the existing static branches' names, > > but I am fine with ibpb_on_vcpu_load if others don't object. > > I assumed as much :-) I'm ok with vcpu_load_ibpb if that's what others prefer. To be honest looking at this again I think I prefer consistency, so if you don't mind and others don't chime in I'd rather keep it as-is. Alternatively I can rename all the static branches (e.g. ibpb_always_on_switch_mm) :P
On Wed, Feb 26, 2025, Yosry Ahmed wrote: > On Tue, Feb 25, 2025 at 02:40:24PM -0800, Sean Christopherson wrote: > > On Tue, Feb 25, 2025, Yosry Ahmed wrote: > > > February 25, 2025 at 11:49 AM, "Sean Christopherson" <seanjc@google.com> wrote: > > > > > > > > On Wed, Feb 19, 2025, Yosry Ahmed wrote: > > > > > > > > > > Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in the > > > > > vCPU load path, introduce a static branch, similar to switch_mm_*_ibpb. > > > > > > > > > > This makes it obvious in spectre_v2_user_select_mitigation() what > > > > > exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB > > > > > (which will be shortly removed). It also provides more fine-grained > > > > > control, making it simpler to change/add paths that control the IBPB in > > > > > the vCPU load path without affecting other IBPBs. > > > > > > > > > > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev> > > > > > > > > > > --- > > > > > > > > > > arch/x86/include/asm/nospec-branch.h | 2 ++ > > > > > arch/x86/kernel/cpu/bugs.c | 5 +++++ > > > > > arch/x86/kvm/svm/svm.c | 2 +- > > > > > arch/x86/kvm/vmx/vmx.c | 2 +- > > > > > 4 files changed, 9 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > > > > > index 7cbb76a2434b9..a22836c5fb338 100644 > > > > > --- a/arch/x86/include/asm/nospec-branch.h > > > > > +++ b/arch/x86/include/asm/nospec-branch.h > > > > > @@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp); > > > > > DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb); > > > > > > > > DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb); > > > > > > > > +DECLARE_STATIC_KEY_FALSE(vcpu_load_ibpb); > > > > > > > > > > > > > How about ibpb_on_vcpu_load? To make it easy for readers to understand exactly > > > > what the knob controls. > > > > > > I was trying to remain consistent with the existing static branches' names, > > > but I am fine with ibpb_on_vcpu_load if others don't object. > > > > I assumed as much :-) I'm ok with vcpu_load_ibpb if that's what others prefer. > > To be honest looking at this again I think I prefer consistency, so if > you don't mind and others don't chime in I'd rather keep it as-is. Works for me. Actually, looking at the names again, wouldn't "switch_vcpu_ibpb" be better? KVM doesn't do IBPB on every vCPU load or even every VMCS load, only when a different vCPU is being loaded. > Alternatively I can rename all the static branches (e.g. > ibpb_always_on_switch_mm) :P LOL, also works for me.
On Wed, Feb 26, 2025 at 04:46:06PM -0800, Sean Christopherson wrote: > On Wed, Feb 26, 2025, Yosry Ahmed wrote: > > On Tue, Feb 25, 2025 at 02:40:24PM -0800, Sean Christopherson wrote: > > > On Tue, Feb 25, 2025, Yosry Ahmed wrote: > > > > February 25, 2025 at 11:49 AM, "Sean Christopherson" <seanjc@google.com> wrote: > > > > > > > > > > On Wed, Feb 19, 2025, Yosry Ahmed wrote: > > > > > > > > > > > > Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in the > > > > > > vCPU load path, introduce a static branch, similar to switch_mm_*_ibpb. > > > > > > > > > > > > This makes it obvious in spectre_v2_user_select_mitigation() what > > > > > > exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB > > > > > > (which will be shortly removed). It also provides more fine-grained > > > > > > control, making it simpler to change/add paths that control the IBPB in > > > > > > the vCPU load path without affecting other IBPBs. > > > > > > > > > > > > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev> > > > > > > > > > > > > --- > > > > > > > > > > > > arch/x86/include/asm/nospec-branch.h | 2 ++ > > > > > > arch/x86/kernel/cpu/bugs.c | 5 +++++ > > > > > > arch/x86/kvm/svm/svm.c | 2 +- > > > > > > arch/x86/kvm/vmx/vmx.c | 2 +- > > > > > > 4 files changed, 9 insertions(+), 2 deletions(-) > > > > > > > > > > > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > > > > > > index 7cbb76a2434b9..a22836c5fb338 100644 > > > > > > --- a/arch/x86/include/asm/nospec-branch.h > > > > > > +++ b/arch/x86/include/asm/nospec-branch.h > > > > > > @@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp); > > > > > > DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb); > > > > > > > > > > DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb); > > > > > > > > > > +DECLARE_STATIC_KEY_FALSE(vcpu_load_ibpb); > > > > > > > > > > > > > > > > How about ibpb_on_vcpu_load? To make it easy for readers to understand exactly > > > > > what the knob controls. > > > > > > > > I was trying to remain consistent with the existing static branches' names, > > > > but I am fine with ibpb_on_vcpu_load if others don't object. > > > > > > I assumed as much :-) I'm ok with vcpu_load_ibpb if that's what others prefer. > > > > To be honest looking at this again I think I prefer consistency, so if > > you don't mind and others don't chime in I'd rather keep it as-is. > > Works for me. > > Actually, looking at the names again, wouldn't "switch_vcpu_ibpb" be better? > KVM doesn't do IBPB on every vCPU load or even every VMCS load, only when a > different vCPU is being loaded. Sold :) > > > Alternatively I can rename all the static branches (e.g. > > ibpb_always_on_switch_mm) :P > > LOL, also works for me.
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