Re: [PATCH 1/2] RISC-V: KVM: Disable vstimecmp before exiting to user-space

[Atish Patra <atish.patra@linux.dev>]

On 7/4/25 8:38 AM, Anup Patel wrote:
> If VS-timer expires when no VCPU running on a host CPU then WFI
> executed by such host CPU will be effective NOP resulting in no
> power savings. This is as-per RISC-V Privileged specificaiton
> which says: "WFI is also required to resume execution for locally
> enabled interrupts pending at any privilege level, regardless of
> the global interrupt enable at each privilege level."
>
> To address the above issue, vstimecmp CSR must be set to -1UL over
> here when VCPU is scheduled-out or exits to user space.
>
> Fixes: 8f5cb44b1bae ("RISC-V: KVM: Support sstc extension")
> Fixes: cea8896bd936 ("RISC-V: KVM: Fix kvm_riscv_vcpu_timer_pending() for Sstc")
> Reported-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2112578
> Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> ---
>   arch/riscv/kvm/vcpu_timer.c | 16 ++++++++++++++++
>   1 file changed, 16 insertions(+)
>
> diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c
> index ff672fa71fcc..85a7262115e1 100644
> --- a/arch/riscv/kvm/vcpu_timer.c
> +++ b/arch/riscv/kvm/vcpu_timer.c
> @@ -345,8 +345,24 @@ void kvm_riscv_vcpu_timer_save(struct kvm_vcpu *vcpu)
>   	/*
>   	 * The vstimecmp CSRs are saved by kvm_riscv_vcpu_timer_sync()
>   	 * upon every VM exit so no need to save here.
> +	 *
> +	 * If VS-timer expires when no VCPU running on a host CPU then
> +	 * WFI executed by such host CPU will be effective NOP resulting
> +	 * in no power savings. This is because as-per RISC-V Privileged
> +	 * specificaiton: "WFI is also required to resume execution for
> +	 * locally enabled interrupts pending at any privilege level,
> +	 * regardless of the global interrupt enable at each privilege
> +	 * level."
> +	 *
> +	 * To address the above issue, vstimecmp CSR must be set to -1UL
> +	 * over here when VCPU is scheduled-out or exits to user space.
>   	 */
>   
> +	csr_write(CSR_VSTIMECMP, -1UL);
> +#if defined(CONFIG_32BIT)
> +	csr_write(CSR_VSTIMECMPH, -1UL);
> +#endif
> +
>   	/* timer should be enabled for the remaining operations */
>   	if (unlikely(!t->init_done))
>   		return;
Reviewed-by: Atish Patra <atishp@rivosinc.com>