[PATCH v3] RISC-V: KVM: Remove scounteren initialization

[Anup Patel]

On Fri, May 16, 2025 at 4:41 AM Atish Patra <atishp at rivosinc.com> wrote:
>
> Scounteren CSR controls the direct access the hpmcounters and cycle/
> instret/time from the userspace. It's the supervisor's responsibility
> to set it up correctly for it's user space. They hypervisor doesn't
> need to decide the policy on behalf of the supervisor.
>
> Signed-off-by: Atish Patra <atishp at rivosinc.com>
> ---
> Changes in v3:
> - Removed the redundant declaration
> - Link to v2: https://lore.kernel.org/r/20250515-fix_scounteren_vs-v2-1-1fd8dc0693e8@rivosinc.com
>
> Changes in v2:
> - Remove the scounteren initialization instead of just setting the TM bit.
> - Link to v1: https://lore.kernel.org/r/20250513-fix_scounteren_vs-v1-1-c1f52af93c79@rivosinc.com
> ---
>  arch/riscv/kvm/vcpu.c | 4 ----
>  1 file changed, 4 deletions(-)
>
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index 60d684c76c58..9bfaae9a11ea 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -111,7 +111,6 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>  {
>         int rc;
>         struct kvm_cpu_context *cntx;
> -       struct kvm_vcpu_csr *reset_csr = &vcpu->arch.guest_reset_csr;
>
>         spin_lock_init(&vcpu->arch.mp_state_lock);
>
> @@ -146,9 +145,6 @@ 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;
> -
>         /* Setup VCPU timer */
>         kvm_riscv_vcpu_timer_init(vcpu);
>
>
> ---
> base-commit: 01f95500a162fca88cefab9ed64ceded5afabc12
> change-id: 20250513-fix_scounteren_vs-fdd86255c7b7
> --

Overall, this looks good.

Reviewed-by: Anup Patel <anup at brainfault.org>

Currently, the scounteren.TM bit is only set by the Linux SBI PMU
driver but KVM RISC-V only provides SBI PMU for guest when
Sscofpmf is available in host so we need the below hunk to
completely get rid-off scounteren initialization in KVM RISC-V.

diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 356d5397b2a2..bdf3352acf4c 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -131,6 +131,12 @@ secondary_start_sbi:
     csrw CSR_IE, zero
     csrw CSR_IP, zero

+#ifndef CONFIG_RISCV_M_MODE
+    /* Enable time CSR */
+    li t0, 0x2
+    csrw CSR_SCOUNTEREN, t0
+#endif
+
     /* Load the global pointer */
     load_global_pointer

@@ -226,6 +232,10 @@ SYM_CODE_START(_start_kernel)
      * to hand it to us.
      */
     csrr a0, CSR_MHARTID
+#else
+    /* Enable time CSR */
+    li t0, 0x2
+    csrw CSR_SCOUNTEREN, t0
 #endif /* CONFIG_RISCV_M_MODE */

     /* Load the global pointer */

I have queued this patch for Linux-6.16 with the above hunk squashed.

Thanks,
Anup