[PATCH v3 0/4] KVM: arm64: Assorted PMU emulation fixes

[PATCH v3 0/4] KVM: arm64: Assorted PMU emulation fixes

[Marc Zyngier]

I recently came across a number of PMU emulation bugs, all which can
result in unexpected behaviours in an unsuspecting guest. The first
two patches already have been discussed on the list, but I'm including
them here as part of a slightly longer series.

The third patch is new as of v2, and fixes a bug preventing chained
events from ever being used.

The last patch fixes an issue that has been here from day one, where
we confuse architectural overflow of a counter and perf sampling
period, and uses a terrible hack^W^W creative way to interact with the
underlying PMU driver so that we can reload the period when handling
the overflow.

* From v2 [1]
  - Dropped PMUv3 patch and moved the logic into the KVM code
  - Properly use UPDATE/RELOAD
  - Collected Andrew's RB

[1] https://lore.kernel.org/kvmarm/20191008160128.8872-1-maz@kernel.org/

Marc Zyngier (4):
  KVM: arm64: pmu: Fix cycle counter truncation
  arm64: KVM: Handle PMCR_EL0.LC as RES1 on pure AArch64 systems
  KVM: arm64: pmu: Set the CHAINED attribute before creating the
    in-kernel event
  KVM: arm64: pmu: Reset sample period on overflow handling

 arch/arm64/kvm/sys_regs.c |  4 ++++
 virt/kvm/arm/pmu.c        | 48 ++++++++++++++++++++++++++++-----------
 2 files changed, 39 insertions(+), 13 deletions(-)

-- 
2.20.1

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[PATCH v3 1/4] KVM: arm64: pmu: Fix cycle counter truncation

[Marc Zyngier]

When a counter is disabled, its value is sampled before the event
is being disabled, and the value written back in the shadow register.

In that process, the value gets truncated to 32bit, which is adequate
for any counter but the cycle counter (defined as a 64bit counter).

This obviously results in a corrupted counter, and things like
"perf record -e cycles" not working at all when run in a guest...
A similar, but less critical bug exists in kvm_pmu_get_counter_value.

Make the truncation conditional on the counter not being the cycle
counter, which results in a minor code reorganisation.

Fixes: 80f393a23be6 ("KVM: arm/arm64: Support chained PMU counters")
Reviewed-by: Andrew Murray <andrew.murray@arm.com>
Reported-by: Julien Thierry <julien.thierry.kdev@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 virt/kvm/arm/pmu.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
index 362a01886bab..c30c3a74fc7f 100644
--- a/virt/kvm/arm/pmu.c
+++ b/virt/kvm/arm/pmu.c
@@ -146,8 +146,7 @@ u64 kvm_pmu_get_counter_value(struct kvm_vcpu *vcpu, u64 select_idx)
 	if (kvm_pmu_pmc_is_chained(pmc) &&
 	    kvm_pmu_idx_is_high_counter(select_idx))
 		counter = upper_32_bits(counter);
-
-	else if (!kvm_pmu_idx_is_64bit(vcpu, select_idx))
+	else if (select_idx != ARMV8_PMU_CYCLE_IDX)
 		counter = lower_32_bits(counter);
 
 	return counter;
@@ -193,7 +192,7 @@ static void kvm_pmu_release_perf_event(struct kvm_pmc *pmc)
  */
 static void kvm_pmu_stop_counter(struct kvm_vcpu *vcpu, struct kvm_pmc *pmc)
 {
-	u64 counter, reg;
+	u64 counter, reg, val;
 
 	pmc = kvm_pmu_get_canonical_pmc(pmc);
 	if (!pmc->perf_event)
@@ -201,16 +200,19 @@ static void kvm_pmu_stop_counter(struct kvm_vcpu *vcpu, struct kvm_pmc *pmc)
 
 	counter = kvm_pmu_get_pair_counter_value(vcpu, pmc);
 
-	if (kvm_pmu_pmc_is_chained(pmc)) {
-		reg = PMEVCNTR0_EL0 + pmc->idx;
-		__vcpu_sys_reg(vcpu, reg) = lower_32_bits(counter);
-		__vcpu_sys_reg(vcpu, reg + 1) = upper_32_bits(counter);
+	if (pmc->idx == ARMV8_PMU_CYCLE_IDX) {
+		reg = PMCCNTR_EL0;
+		val = counter;
 	} else {
-		reg = (pmc->idx == ARMV8_PMU_CYCLE_IDX)
-		       ? PMCCNTR_EL0 : PMEVCNTR0_EL0 + pmc->idx;
-		__vcpu_sys_reg(vcpu, reg) = lower_32_bits(counter);
+		reg = PMEVCNTR0_EL0 + pmc->idx;
+		val = lower_32_bits(counter);
 	}
 
+	__vcpu_sys_reg(vcpu, reg) = val;
+
+	if (kvm_pmu_pmc_is_chained(pmc))
+		__vcpu_sys_reg(vcpu, reg + 1) = upper_32_bits(counter);
+
 	kvm_pmu_release_perf_event(pmc);
 }
 
-- 
2.20.1

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[PATCH v3 2/4] arm64: KVM: Handle PMCR_EL0.LC as RES1 on pure AArch64 systems

[Marc Zyngier]

Of PMCR_EL0.LC, the ARMv8 ARM says:

	"In an AArch64 only implementation, this field is RES 1."

So be it.

Fixes: ab9468340d2bc ("arm64: KVM: Add access handler for PMCR register")
Reviewed-by: Andrew Murray <andrew.murray@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/sys_regs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 2071260a275b..46822afc57e0 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -632,6 +632,8 @@ static void reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 	 */
 	val = ((pmcr & ~ARMV8_PMU_PMCR_MASK)
 	       | (ARMV8_PMU_PMCR_MASK & 0xdecafbad)) & (~ARMV8_PMU_PMCR_E);
+	if (!system_supports_32bit_el0())
+		val |= ARMV8_PMU_PMCR_LC;
 	__vcpu_sys_reg(vcpu, r->reg) = val;
 }
 
@@ -682,6 +684,8 @@ static bool access_pmcr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
 		val = __vcpu_sys_reg(vcpu, PMCR_EL0);
 		val &= ~ARMV8_PMU_PMCR_MASK;
 		val |= p->regval & ARMV8_PMU_PMCR_MASK;
+		if (!system_supports_32bit_el0())
+			val |= ARMV8_PMU_PMCR_LC;
 		__vcpu_sys_reg(vcpu, PMCR_EL0) = val;
 		kvm_pmu_handle_pmcr(vcpu, val);
 		kvm_vcpu_pmu_restore_guest(vcpu);
-- 
2.20.1

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[PATCH v3 3/4] KVM: arm64: pmu: Set the CHAINED attribute before creating the in-kernel event

[Marc Zyngier]

The current convention for KVM to request a chained event from the
host PMU is to set bit[0] in attr.config1 (PERF_ATTR_CFG1_KVM_PMU_CHAINED).

But as it turns out, this bit gets set *after* we create the kernel
event that backs our virtual counter, meaning that we never get
a 64bit counter.

Moving the setting to an earlier point solves the problem.

Fixes: 80f393a23be6 ("KVM: arm/arm64: Support chained PMU counters")
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 virt/kvm/arm/pmu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
index c30c3a74fc7f..f291d4ac3519 100644
--- a/virt/kvm/arm/pmu.c
+++ b/virt/kvm/arm/pmu.c
@@ -569,12 +569,12 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
 		 * high counter.
 		 */
 		attr.sample_period = (-counter) & GENMASK(63, 0);
+		if (kvm_pmu_counter_is_enabled(vcpu, pmc->idx + 1))
+			attr.config1 |= PERF_ATTR_CFG1_KVM_PMU_CHAINED;
+
 		event = perf_event_create_kernel_counter(&attr, -1, current,
 							 kvm_pmu_perf_overflow,
 							 pmc + 1);
-
-		if (kvm_pmu_counter_is_enabled(vcpu, pmc->idx + 1))
-			attr.config1 |= PERF_ATTR_CFG1_KVM_PMU_CHAINED;
 	} else {
 		/* The initial sample period (overflow count) of an event. */
 		if (kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
-- 
2.20.1

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[PATCH v3 4/4] KVM: arm64: pmu: Reset sample period on overflow handling

[Marc Zyngier]

The PMU emulation code uses the perf event sample period to trigger
the overflow detection. This works fine  for the *first* overflow
handling, but results in a huge number of interrupts on the host,
unrelated to the number of interrupts handled in the guest (a x20
factor is pretty common for the cycle counter). On a slow system
(such as a SW model), this can result in the guest only making
forward progress at a glacial pace.

It turns out that the clue is in the name. The sample period is
exactly that: a period. And once the an overflow has occured,
the following period should be the full width of the associated
counter, instead of whatever the guest had initially programed.

Reset the sample period to the architected value in the overflow
handler, which now results in a number of host interrupts that is
much closer to the number of interrupts in the guest.

Fixes: b02386eb7dac ("arm64: KVM: Add PMU overflow interrupt routing")
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 virt/kvm/arm/pmu.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
index f291d4ac3519..8731dfeced8b 100644
--- a/virt/kvm/arm/pmu.c
+++ b/virt/kvm/arm/pmu.c
@@ -8,6 +8,7 @@
 #include <linux/kvm.h>
 #include <linux/kvm_host.h>
 #include <linux/perf_event.h>
+#include <linux/perf/arm_pmu.h>
 #include <linux/uaccess.h>
 #include <asm/kvm_emulate.h>
 #include <kvm/arm_pmu.h>
@@ -442,8 +443,25 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
 				  struct pt_regs *regs)
 {
 	struct kvm_pmc *pmc = perf_event->overflow_handler_context;
+	struct arm_pmu *cpu_pmu = to_arm_pmu(perf_event->pmu);
 	struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
 	int idx = pmc->idx;
+	u64 period;
+
+	cpu_pmu->pmu.stop(perf_event, PERF_EF_UPDATE);
+
+	/*
+	 * Reset the sample period to the architectural limit,
+	 * i.e. the point where the counter overflows.
+	 */
+	period = -(local64_read(&perf_event->count));
+
+	if (!kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
+		period &= GENMASK(31, 0);
+
+	local64_set(&perf_event->hw.period_left, 0);
+	perf_event->attr.sample_period = period;
+	perf_event->hw.sample_period = period;
 
 	__vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(idx);
 
@@ -451,6 +469,8 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
 		kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
 		kvm_vcpu_kick(vcpu);
 	}
+
+	cpu_pmu->pmu.start(perf_event, PERF_EF_RELOAD);
 }
 
 /**
-- 
2.20.1

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Re: [PATCH v3 3/4] KVM: arm64: pmu: Set the CHAINED attribute before creating the in-kernel event

[Andrew Murray]

On Fri, Oct 11, 2019 at 01:39:53PM +0100, Marc Zyngier wrote:
> The current convention for KVM to request a chained event from the
> host PMU is to set bit[0] in attr.config1 (PERF_ATTR_CFG1_KVM_PMU_CHAINED).
> 
> But as it turns out, this bit gets set *after* we create the kernel
> event that backs our virtual counter, meaning that we never get
> a 64bit counter.
> 
> Moving the setting to an earlier point solves the problem.
> 
> Fixes: 80f393a23be6 ("KVM: arm/arm64: Support chained PMU counters")
> Signed-off-by: Marc Zyngier <maz@kernel.org>

Reviewed-by: Andrew Murray <andrew.murray@arm.com>

> ---
>  virt/kvm/arm/pmu.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> index c30c3a74fc7f..f291d4ac3519 100644
> --- a/virt/kvm/arm/pmu.c
> +++ b/virt/kvm/arm/pmu.c
> @@ -569,12 +569,12 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
>  		 * high counter.
>  		 */
>  		attr.sample_period = (-counter) & GENMASK(63, 0);
> +		if (kvm_pmu_counter_is_enabled(vcpu, pmc->idx + 1))
> +			attr.config1 |= PERF_ATTR_CFG1_KVM_PMU_CHAINED;
> +
>  		event = perf_event_create_kernel_counter(&attr, -1, current,
>  							 kvm_pmu_perf_overflow,
>  							 pmc + 1);
> -
> -		if (kvm_pmu_counter_is_enabled(vcpu, pmc->idx + 1))
> -			attr.config1 |= PERF_ATTR_CFG1_KVM_PMU_CHAINED;
>  	} else {
>  		/* The initial sample period (overflow count) of an event. */
>  		if (kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
> -- 
> 2.20.1
> 
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Re: [PATCH v3 4/4] KVM: arm64: pmu: Reset sample period on overflow handling

[Andrew Murray]

On Fri, Oct 11, 2019 at 01:39:54PM +0100, Marc Zyngier wrote:
> The PMU emulation code uses the perf event sample period to trigger
> the overflow detection. This works fine  for the *first* overflow
> handling, but results in a huge number of interrupts on the host,
> unrelated to the number of interrupts handled in the guest (a x20
> factor is pretty common for the cycle counter). On a slow system
> (such as a SW model), this can result in the guest only making
> forward progress at a glacial pace.
> 
> It turns out that the clue is in the name. The sample period is
> exactly that: a period. And once the an overflow has occured,
> the following period should be the full width of the associated
> counter, instead of whatever the guest had initially programed.
> 
> Reset the sample period to the architected value in the overflow
> handler, which now results in a number of host interrupts that is
> much closer to the number of interrupts in the guest.
> 
> Fixes: b02386eb7dac ("arm64: KVM: Add PMU overflow interrupt routing")
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---

Reviewed-by: Andrew Murray <andrew.murray@arm.com>

>  virt/kvm/arm/pmu.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
> index f291d4ac3519..8731dfeced8b 100644
> --- a/virt/kvm/arm/pmu.c
> +++ b/virt/kvm/arm/pmu.c
> @@ -8,6 +8,7 @@
>  #include <linux/kvm.h>
>  #include <linux/kvm_host.h>
>  #include <linux/perf_event.h>
> +#include <linux/perf/arm_pmu.h>
>  #include <linux/uaccess.h>
>  #include <asm/kvm_emulate.h>
>  #include <kvm/arm_pmu.h>
> @@ -442,8 +443,25 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
>  				  struct pt_regs *regs)
>  {
>  	struct kvm_pmc *pmc = perf_event->overflow_handler_context;
> +	struct arm_pmu *cpu_pmu = to_arm_pmu(perf_event->pmu);
>  	struct kvm_vcpu *vcpu = kvm_pmc_to_vcpu(pmc);
>  	int idx = pmc->idx;
> +	u64 period;
> +
> +	cpu_pmu->pmu.stop(perf_event, PERF_EF_UPDATE);
> +
> +	/*
> +	 * Reset the sample period to the architectural limit,
> +	 * i.e. the point where the counter overflows.
> +	 */
> +	period = -(local64_read(&perf_event->count));
> +
> +	if (!kvm_pmu_idx_is_64bit(vcpu, pmc->idx))
> +		period &= GENMASK(31, 0);
> +
> +	local64_set(&perf_event->hw.period_left, 0);
> +	perf_event->attr.sample_period = period;
> +	perf_event->hw.sample_period = period;
>  
>  	__vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(idx);
>  
> @@ -451,6 +469,8 @@ static void kvm_pmu_perf_overflow(struct perf_event *perf_event,
>  		kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);
>  		kvm_vcpu_kick(vcpu);
>  	}
> +
> +	cpu_pmu->pmu.start(perf_event, PERF_EF_RELOAD);
>  }
>  
>  /**
> -- 
> 2.20.1
> 
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