linux-kernel - [PATCH] RISC-V: KVM: Disable instret/cycle for VU mode by default

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Message-Id: <20250513-fix_scounteren_vs-v1-1-c1f52af93c79@rivosinc.com>
Date: Tue, 13 May 2025 23:43:30 -0700
From: Atish Patra <atishp@...osinc.com>
To: Anup Patel <anup@...infault.org>, Atish Patra <atishp@...shpatra.org>, 
 Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Alexandre Ghiti <alex@...ti.fr>
Cc: kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org, 
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Atish Patra <atishp@...osinc.com>
Subject: [PATCH] RISC-V: KVM: Disable instret/cycle for VU mode by default

The KVM virtualizes PMU in RISC-V and disables all counter access except
TM bit by default vi hstateen CSR. There is no benefit in enabling CY/TM
bits in scounteren for the guest user space as it can't be run without
hcounteren anyways.

Allow only TM bit which matches the hcounteren default setting.

Signed-off-by: Atish Patra <atishp@...osinc.com>
---
 arch/riscv/kvm/vcpu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
index 60d684c76c58..873593bfe610 100644
--- a/arch/riscv/kvm/vcpu.c
+++ b/arch/riscv/kvm/vcpu.c
@@ -146,8 +146,8 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 	if (kvm_riscv_vcpu_alloc_vector_context(vcpu, cntx))
 		return -ENOMEM;
 
-	/* By default, make CY, TM, and IR counters accessible in VU mode */
-	reset_csr->scounteren = 0x7;
+	/* By default, only TM should be accessible in VU mode */
+	reset_csr->scounteren = 0x2;
 
 	/* Setup VCPU timer */
 	kvm_riscv_vcpu_timer_init(vcpu);

---
base-commit: 01f95500a162fca88cefab9ed64ceded5afabc12
change-id: 20250513-fix_scounteren_vs-fdd86255c7b7
--
Regards,
Atish patra