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From: Marc Zyngier <maz@kernel.org>
To: linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu
Cc: kernel-team@android.com
Subject: [PATCH] KVM: arm64: Don't access PMCR_EL0 when no PMU is available
Date: Thu, 10 Dec 2020 08:30:59 +0000	[thread overview]
Message-ID: <20201210083059.1277162-1-maz@kernel.org> (raw)

We reset the guest's view of PMCR_EL0 unconditionally, based on
the host's view of this register. It is however legal for an
imnplementation not to provide any PMU, resulting in an UNDEF.

The obvious fix is to skip the reset of this shadow register
when no PMU is available, sidestepping the issue entirely.
If no PMU is available, the guest is not able to request
a virtual PMU anyway, so not doing nothing is the right thing
to do!

It is unlikely that this bug can hit any HW implementation
though, as they all provide a PMU. It has been found using nested
virt with the host KVM not implementing the PMU itself.

Fixes: ab9468340d2bc ("arm64: KVM: Add access handler for PMCR register")
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 bc15246775d0..6c64d010102b 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -923,6 +923,10 @@ static void reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	u64 pmcr, val;
 
+	/* No PMU available, PMCR_EL0 may UNDEF... */
+	if (!kvm_arm_support_pmu_v3())
+		return;
+
 	pmcr = read_sysreg(pmcr_el0);
 	/*
 	 * Writable bits of PMCR_EL0 (ARMV8_PMU_PMCR_MASK) are reset to UNKNOWN
-- 
2.29.2

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kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

WARNING: multiple messages have this Message-ID (diff)
From: Marc Zyngier <maz@kernel.org>
To: linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu
Cc: James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	kernel-team@android.com,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>
Subject: [PATCH] KVM: arm64: Don't access PMCR_EL0 when no PMU is available
Date: Thu, 10 Dec 2020 08:30:59 +0000	[thread overview]
Message-ID: <20201210083059.1277162-1-maz@kernel.org> (raw)

We reset the guest's view of PMCR_EL0 unconditionally, based on
the host's view of this register. It is however legal for an
imnplementation not to provide any PMU, resulting in an UNDEF.

The obvious fix is to skip the reset of this shadow register
when no PMU is available, sidestepping the issue entirely.
If no PMU is available, the guest is not able to request
a virtual PMU anyway, so not doing nothing is the right thing
to do!

It is unlikely that this bug can hit any HW implementation
though, as they all provide a PMU. It has been found using nested
virt with the host KVM not implementing the PMU itself.

Fixes: ab9468340d2bc ("arm64: KVM: Add access handler for PMCR register")
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 bc15246775d0..6c64d010102b 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -923,6 +923,10 @@ static void reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
 	u64 pmcr, val;
 
+	/* No PMU available, PMCR_EL0 may UNDEF... */
+	if (!kvm_arm_support_pmu_v3())
+		return;
+
 	pmcr = read_sysreg(pmcr_el0);
 	/*
 	 * Writable bits of PMCR_EL0 (ARMV8_PMU_PMCR_MASK) are reset to UNKNOWN
-- 
2.29.2


_______________________________________________
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linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

             reply	other threads:[~2020-12-10  8:31 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-10  8:30 Marc Zyngier [this message]
2020-12-10  8:30 ` [PATCH] KVM: arm64: Don't access PMCR_EL0 when no PMU is available Marc Zyngier
2020-12-10 10:12 ` Alexandru Elisei
2020-12-10 10:12   ` Alexandru Elisei
2020-12-10 11:16   ` Marc Zyngier
2020-12-10 11:16     ` Marc Zyngier
2020-12-10 12:22     ` Alexandru Elisei
2020-12-10 12:22       ` Alexandru Elisei
2021-01-04 15:47 ` Qian Cai
2021-01-04 15:47   ` Qian Cai
2021-01-04 15:47   ` Qian Cai
2021-01-04 16:08   ` Marc Zyngier
2021-01-04 16:08     ` Marc Zyngier
2021-01-04 16:08     ` Marc Zyngier
2021-01-04 16:22     ` Qian Cai
2021-01-04 16:22       ` Qian Cai
2021-01-04 16:22       ` Qian Cai
2021-01-04 16:27       ` Marc Zyngier
2021-01-04 16:27         ` Marc Zyngier
2021-01-04 16:27         ` Marc Zyngier
2021-01-04 18:20         ` Qian Cai
2021-01-04 18:20           ` Qian Cai
2021-01-04 18:20           ` Qian Cai
2021-01-04 18:26           ` Marc Zyngier
2021-01-04 18:26             ` Marc Zyngier
2021-01-04 18:26             ` Marc Zyngier
2021-01-04 18:42             ` Qian Cai
2021-01-04 18:42               ` Qian Cai
2021-01-04 18:42               ` Qian Cai
2021-01-04 19:32               ` Marc Zyngier
2021-01-04 19:32                 ` Marc Zyngier
2021-01-04 19:32                 ` Marc Zyngier

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