Re: [PATCH v5] KVM: riscv: Skip CSR restore if VCPU is reloaded on the same core

[Andrew Jones]

On Wed, Feb 25, 2026 at 11:14:02AM +0800, Jinyu Tang wrote:
>  	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
>  	struct kvm_vcpu_config *cfg = &vcpu->arch.cfg;
>  
> +	/*
> +	 * If VCPU is being reloaded on the same physical CPU and no
> +	 * other KVM VCPU has run on this CPU since it was last put,
> +	 * we can skip the expensive CSR and HGATP writes.
> +	 *
> +	 * Note: If a new CSR is added to this fast-path skip block,
> +	 * make sure that 'csr_dirty' is set to true in any
> +	 * ioctl (e.g., KVM_SET_ONE_REG) that modifies it.
> +	 */
> +	if (vcpu == __this_cpu_read(kvm_former_vcpu) &&
> +	    vcpu->arch.last_exit_cpu == cpu && !vcpu->arch.csr_dirty)

The most expensive check condition, __this_cpu_read(), should come last
in order to short-circuit it when either of the cheap checks fail.

> +		goto csr_restore_done;
> +
> +	vcpu->arch.csr_dirty = false;
>  	if (kvm_riscv_nacl_sync_csr_available()) {
>  		nsh = nacl_shmem();
>  		nacl_csr_write(nsh, CSR_VSSTATUS, csr->vsstatus);
> @@ -624,6 +642,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  
>  	kvm_riscv_mmu_update_hgatp(vcpu);
>  
> +csr_restore_done:
>  	kvm_riscv_vcpu_timer_restore(vcpu);
>  
>  	kvm_riscv_vcpu_host_fp_save(&vcpu->arch.host_context);
> @@ -645,6 +664,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  	void *nsh;
>  	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
>  
> +	__this_cpu_write(kvm_former_vcpu, vcpu);

This write can be skipped by the fast path since kvm_former_vcpu is
already set to vcpu.

Thanks,
drew