[PATCH 0/3] Pin the hrtimer used for VMX-preemption timer emulation

[PATCH 0/3] Pin the hrtimer used for VMX-preemption timer emulation

[Jim Mattson]

I'm still not entirely convinced that the Linux hrtimer can be used to
accurately emulate the VMX-preemption timer, but it definitely doesn't
work if the hrtimer is on a different logical processor from the vCPU
thread that needs to get kicked out of VMX non-root operation.

With these changes, the kvm-unit-test (sent separately) that verifies
that a guest can't actually observe a delayed VMX-preemption timer
VM-exit passes 99.999% of the time on a 2GHz Skylake system.

It might be possible to improve that pass rate even more by increasing
the scaling factor in the virtual IA32_VMX_MISC[4:0], but you'd have
to be even more of a stickler than I to go to that extreme.

By the way, what is the point of migrating the hrtimers for the APIC
and the PIT, since they aren't even pinned to begin with?

The subject line of the first patch was crafted for you, Sean. :-D

Jim Mattson (3):
  KVM: nVMX: Really make emulated nested preemption timer pinned
  KVM: nVMX: Change emulated VMX-preemption timer hrtimer to absolute
  KVM: nVMX: Migrate the VMX-preemption timer

 arch/x86/include/asm/kvm_host.h |  2 ++
 arch/x86/kvm/irq.c              |  2 ++
 arch/x86/kvm/vmx/nested.c       |  5 +++--
 arch/x86/kvm/vmx/vmx.c          | 11 +++++++++++
 4 files changed, 18 insertions(+), 2 deletions(-)

-- 
2.26.2.645.ge9eca65c58-goog

[PATCH 1/3] KVM: nVMX: Really make emulated nested preemption timer pinned

[Jim Mattson]

The PINNED bit is ignored by hrtimer_init. It is only considered when
starting the timer.

When the hrtimer isn't pinned to the same logical processor as the
vCPU thread to be interrupted, the emulated VMX-preemption timer
often fails to adhere to the architectural specification.

Fixes: f15a75eedc18e ("KVM: nVMX: make emulated nested preemption timer pinned")
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
---
 arch/x86/kvm/vmx/nested.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index fd78ffbde644..1f7fe6f47cbc 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2041,7 +2041,7 @@ static void vmx_start_preemption_timer(struct kvm_vcpu *vcpu)
 	preemption_timeout *= 1000000;
 	do_div(preemption_timeout, vcpu->arch.virtual_tsc_khz);
 	hrtimer_start(&vmx->nested.preemption_timer,
-		      ns_to_ktime(preemption_timeout), HRTIMER_MODE_REL);
+		      ns_to_ktime(preemption_timeout), HRTIMER_MODE_REL_PINNED);
 }
 
 static u64 nested_vmx_calc_efer(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
-- 
2.26.2.645.ge9eca65c58-goog

Re: [PATCH 1/3] KVM: nVMX: Really make emulated nested preemption timer pinned

[Sean Christopherson]

LOL, love the shortlog :-)

On Fri, May 08, 2020 at 01:36:41PM -0700, Jim Mattson wrote:
> The PINNED bit is ignored by hrtimer_init. It is only considered when
> starting the timer.
> 
> When the hrtimer isn't pinned to the same logical processor as the
> vCPU thread to be interrupted, the emulated VMX-preemption timer
> often fails to adhere to the architectural specification.
> 
> Fixes: f15a75eedc18e ("KVM: nVMX: make emulated nested preemption timer pinned")
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Reviewed-by: Peter Shier <pshier@google.com>
> Reviewed-by: Oliver Upton <oupton@google.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index fd78ffbde644..1f7fe6f47cbc 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2041,7 +2041,7 @@ static void vmx_start_preemption_timer(struct kvm_vcpu *vcpu)
>  	preemption_timeout *= 1000000;
>  	do_div(preemption_timeout, vcpu->arch.virtual_tsc_khz);
>  	hrtimer_start(&vmx->nested.preemption_timer,
> -		      ns_to_ktime(preemption_timeout), HRTIMER_MODE_REL);
> +		      ns_to_ktime(preemption_timeout), HRTIMER_MODE_REL_PINNED);
>  }
>  
>  static u64 nested_vmx_calc_efer(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
> -- 
> 2.26.2.645.ge9eca65c58-goog
> 

[PATCH 2/3] KVM: nVMX: Change emulated VMX-preemption timer hrtimer to absolute

[Jim Mattson]

Prepare for migration of this hrtimer, by changing it from relative to
absolute. (I couldn't get migration to work with a relative timer.)

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
---
 arch/x86/kvm/vmx/nested.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 1f7fe6f47cbc..3ca792e3cd66 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2041,7 +2041,8 @@ static void vmx_start_preemption_timer(struct kvm_vcpu *vcpu)
 	preemption_timeout *= 1000000;
 	do_div(preemption_timeout, vcpu->arch.virtual_tsc_khz);
 	hrtimer_start(&vmx->nested.preemption_timer,
-		      ns_to_ktime(preemption_timeout), HRTIMER_MODE_REL_PINNED);
+		      ktime_add_ns(ktime_get(), preemption_timeout),
+		      HRTIMER_MODE_ABS_PINNED);
 }
 
 static u64 nested_vmx_calc_efer(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
@@ -4614,7 +4615,7 @@ static int enter_vmx_operation(struct kvm_vcpu *vcpu)
 		goto out_shadow_vmcs;
 
 	hrtimer_init(&vmx->nested.preemption_timer, CLOCK_MONOTONIC,
-		     HRTIMER_MODE_REL_PINNED);
+		     HRTIMER_MODE_ABS_PINNED);
 	vmx->nested.preemption_timer.function = vmx_preemption_timer_fn;
 
 	vmx->nested.vpid02 = allocate_vpid();
-- 
2.26.2.645.ge9eca65c58-goog

[PATCH 3/3] KVM: nVMX: Migrate the VMX-preemption timer

[Jim Mattson]

The hrtimer used to emulate the VMX-preemption timer must be pinned to
the same logical processor as the vCPU thread to be interrupted if we
want to have any hope of adhering to the architectural specification
of the VMX-preemption timer. Even with this change, the emulated
VMX-preemption timer VM-exit occasionally arrives too late.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
---
 arch/x86/include/asm/kvm_host.h |  2 ++
 arch/x86/kvm/irq.c              |  2 ++
 arch/x86/kvm/vmx/vmx.c          | 11 +++++++++++
 3 files changed, 15 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 42a2d0d3984a..a47c71d13039 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1254,6 +1254,8 @@ struct kvm_x86_ops {
 
 	bool (*apic_init_signal_blocked)(struct kvm_vcpu *vcpu);
 	int (*enable_direct_tlbflush)(struct kvm_vcpu *vcpu);
+
+	void (*migrate_timers)(struct kvm_vcpu *vcpu);
 };
 
 struct kvm_x86_init_ops {
diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
index e330e7d125f7..54f7ea68083b 100644
--- a/arch/x86/kvm/irq.c
+++ b/arch/x86/kvm/irq.c
@@ -159,6 +159,8 @@ void __kvm_migrate_timers(struct kvm_vcpu *vcpu)
 {
 	__kvm_migrate_apic_timer(vcpu);
 	__kvm_migrate_pit_timer(vcpu);
+	if (kvm_x86_ops.migrate_timers)
+		kvm_x86_ops.migrate_timers(vcpu);
 }
 
 bool kvm_arch_irqfd_allowed(struct kvm *kvm, struct kvm_irqfd *args)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index c2c6335a998c..3896dea72082 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7687,6 +7687,16 @@ static bool vmx_apic_init_signal_blocked(struct kvm_vcpu *vcpu)
 	return to_vmx(vcpu)->nested.vmxon;
 }
 
+static void vmx_migrate_timers(struct kvm_vcpu *vcpu)
+{
+	if (is_guest_mode(vcpu)) {
+		struct hrtimer *timer = &to_vmx(vcpu)->nested.preemption_timer;
+
+		if (hrtimer_try_to_cancel(timer) == 1)
+			hrtimer_start_expires(timer, HRTIMER_MODE_ABS_PINNED);
+	}
+}
+
 static void hardware_unsetup(void)
 {
 	if (nested)
@@ -7838,6 +7848,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
 	.nested_get_evmcs_version = NULL,
 	.need_emulation_on_page_fault = vmx_need_emulation_on_page_fault,
 	.apic_init_signal_blocked = vmx_apic_init_signal_blocked,
+	.migrate_timers = vmx_migrate_timers,
 };
 
 static __init int hardware_setup(void)
-- 
2.26.2.645.ge9eca65c58-goog

Re: [PATCH 3/3] KVM: nVMX: Migrate the VMX-preemption timer

[Paolo Bonzini]

On 08/05/20 22:36, Jim Mattson wrote:
> The hrtimer used to emulate the VMX-preemption timer must be pinned to
> the same logical processor as the vCPU thread to be interrupted if we
> want to have any hope of adhering to the architectural specification
> of the VMX-preemption timer. Even with this change, the emulated
> VMX-preemption timer VM-exit occasionally arrives too late.
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Reviewed-by: Peter Shier <pshier@google.com>
> Reviewed-by: Oliver Upton <oupton@google.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 ++
>  arch/x86/kvm/irq.c              |  2 ++
>  arch/x86/kvm/vmx/vmx.c          | 11 +++++++++++
>  3 files changed, 15 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 42a2d0d3984a..a47c71d13039 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1254,6 +1254,8 @@ struct kvm_x86_ops {
>  
>  	bool (*apic_init_signal_blocked)(struct kvm_vcpu *vcpu);
>  	int (*enable_direct_tlbflush)(struct kvm_vcpu *vcpu);
> +
> +	void (*migrate_timers)(struct kvm_vcpu *vcpu);
>  };
>  
>  struct kvm_x86_init_ops {
> diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
> index e330e7d125f7..54f7ea68083b 100644
> --- a/arch/x86/kvm/irq.c
> +++ b/arch/x86/kvm/irq.c
> @@ -159,6 +159,8 @@ void __kvm_migrate_timers(struct kvm_vcpu *vcpu)
>  {
>  	__kvm_migrate_apic_timer(vcpu);
>  	__kvm_migrate_pit_timer(vcpu);
> +	if (kvm_x86_ops.migrate_timers)
> +		kvm_x86_ops.migrate_timers(vcpu);
>  }
>  
>  bool kvm_arch_irqfd_allowed(struct kvm *kvm, struct kvm_irqfd *args)
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c2c6335a998c..3896dea72082 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7687,6 +7687,16 @@ static bool vmx_apic_init_signal_blocked(struct kvm_vcpu *vcpu)
>  	return to_vmx(vcpu)->nested.vmxon;
>  }
>  
> +static void vmx_migrate_timers(struct kvm_vcpu *vcpu)
> +{
> +	if (is_guest_mode(vcpu)) {
> +		struct hrtimer *timer = &to_vmx(vcpu)->nested.preemption_timer;
> +
> +		if (hrtimer_try_to_cancel(timer) == 1)
> +			hrtimer_start_expires(timer, HRTIMER_MODE_ABS_PINNED);
> +	}
> +}
> +
>  static void hardware_unsetup(void)
>  {
>  	if (nested)
> @@ -7838,6 +7848,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
>  	.nested_get_evmcs_version = NULL,
>  	.need_emulation_on_page_fault = vmx_need_emulation_on_page_fault,
>  	.apic_init_signal_blocked = vmx_apic_init_signal_blocked,
> +	.migrate_timers = vmx_migrate_timers,
>  };
>  
>  static __init int hardware_setup(void)
> 

Queued all, thanks.

Paolo

Re: [PATCH 0/3] Pin the hrtimer used for VMX-preemption timer emulation

[Paolo Bonzini]

On 08/05/20 22:36, Jim Mattson wrote:
> 
> It might be possible to improve that pass rate even more by increasing
> the scaling factor in the virtual IA32_VMX_MISC[4:0], but you'd have
> to be even more of a stickler than I to go to that extreme.
> 
> By the way, what is the point of migrating the hrtimers for the APIC
> and the PIT, since they aren't even pinned to begin with?

Unless housekeeping CPUs for timers are configured, the timer will run
on the CPU it is started on, even without pinning; see
get_nohz_timer_target.

Paolo

Re: [PATCH 0/3] Pin the hrtimer used for VMX-preemption timer emulation

[Jim Mattson]

On Fri, May 8, 2020 at 1:36 PM Jim Mattson <jmattson@google.com> wrote:
>
> I'm still not entirely convinced that the Linux hrtimer can be used to
> accurately emulate the VMX-preemption timer...

It can't, for several reasons:

1) The conversion between wall-clock time and TSC frequency, based on
tsc_khz, isn't precise enough.
2) The base clock for the hrtimer, CLOCK_MONOTONIC, can be slewed,
whereas the TSC cannot.
3) The VMX-preemption timer is suspended during MWAIT; the hrtimer is not.

Is there any reason that VMX-preemption timer emulation shouldn't just
be a second client of the hv_timer, along with lapic timer emulation?

Re: [PATCH 0/3] Pin the hrtimer used for VMX-preemption timer emulation

[Paolo Bonzini]

On 19/06/20 20:49, Jim Mattson wrote:
> On Fri, May 8, 2020 at 1:36 PM Jim Mattson <jmattson@google.com> wrote:
>>
>> I'm still not entirely convinced that the Linux hrtimer can be used to
>> accurately emulate the VMX-preemption timer...
> 
> It can't, for several reasons:
> 
> 1) The conversion between wall-clock time and TSC frequency, based on
> tsc_khz, isn't precise enough.
> 2) The base clock for the hrtimer, CLOCK_MONOTONIC, can be slewed,
> whereas the TSC cannot.
> 3) The VMX-preemption timer is suspended during MWAIT; the hrtimer is not.
> 
> Is there any reason that VMX-preemption timer emulation shouldn't just
> be a second client of the hv_timer, along with lapic timer emulation?

I don't think so, you'd just need logic to multiplex it.

Paolo