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

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Message-ID: <42c6f9d7-ac1a-4f74-8b0b-ac8fb02de0b0@canonical.com>
Date: Fri, 4 Jul 2025 21:46:22 +0200
From: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
To: Anup Patel <apatel@...tanamicro.com>, Atish Patra <atish.patra@...ux.dev>
Cc: Palmer Dabbelt <palmer@...belt.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Alexandre Ghiti <alex@...ti.fr>,
 Andrew Jones <ajones@...tanamicro.com>, Anup Patel <anup@...infault.org>,
 kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Emil Renner Berthing <emil.renner.berthing@...onical.com>
Subject: Re: [PATCH 1/2] RISC-V: KVM: Disable vstimecmp before exiting to
 user-space

On 04.07.25 17:38, 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@...onical.com>
> Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2112578
> Signed-off-by: Anup Patel <apatel@...tanamicro.com>

Anup and Atish, thanks a lot for investigating.

I built upstream kernel 6.14.9 with the patches of this series and that 
resolved the problem reported in Launchpad bug report 2112578.

Tested-by: Heinrich Schuchardt <heinrich.schuchardt@...onical.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;