From 57f576e860d3a3582edb5064db9f0c95558ca5f4 Mon Sep 17 00:00:00 2001 From: Anup Patel Date: Mon, 7 Jul 2025 09:23:43 +0530 Subject: [PATCH] RISC-V: KVM: Disable vstimecmp before exiting to user-space 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. Reviewed-by: Atish Patra Tested-by: Atish Patra Tested-by: Heinrich Schuchardt 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 Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2112578 Signed-off-by: Anup Patel Reviewed-by: Nutty Liu Link: https://lore.kernel.org/r/20250707035345.17494-2-apatel@ventanamicro.com Signed-off-by: Anup Patel --- 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 ff672fa71fccf..85a7262115e13 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; -- 2.47.2