Re: [PATCH] riscv: skip csr restore if vcpu preempted reload

[Andrew Jones <ajones@ventanamicro.com>]

On Thu, Aug 07, 2025 at 07:42:20PM +0800, Jinyu Tang wrote:
> The kvm_arch_vcpu_load() function is called in two cases for riscv:
> 1. When entering KVM_RUN from userspace ioctl.
> 2. When a preempted VCPU is scheduled back.
> 
> In the second case, if no other KVM VCPU has run on this CPU since the
> current VCPU was preempted, the guest CSR values are still valid in
> the hardware and do not need to be restored.
> 
> This patch is to skip the CSR write path when:
> 1. The VCPU was previously preempted
> (vcpu->scheduled_out == 1).
> 2. It is being reloaded on the same physical CPU
> (vcpu->arch.last_exit_cpu == cpu).
> 3. No other KVM VCPU has used this CPU in the meantime
> (vcpu == __this_cpu_read(kvm_former_vcpu)).
> 
> This reduces many CSR writes with frequent preemption on the same CPU.
> 
> Signed-off-by: Jinyu Tang <tjytimi@163.com>
> ---
>  arch/riscv/kvm/vcpu.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index f001e5640..1c6c55ee1 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -25,6 +25,8 @@
>  #define CREATE_TRACE_POINTS
>  #include "trace.h"
>  
> +static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_former_vcpu);
> +
>  const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
>  	KVM_GENERIC_VCPU_STATS(),
>  	STATS_DESC_COUNTER(VCPU, ecall_exit_stat),
> @@ -581,6 +583,10 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
>  	struct kvm_vcpu_config *cfg = &vcpu->arch.cfg;
>  
> +	if (vcpu->scheduled_out && vcpu == __this_cpu_read(kvm_former_vcpu) &&
> +		vcpu->arch.last_exit_cpu == cpu)
> +		goto csr_restore_done;
> +
>  	if (kvm_riscv_nacl_sync_csr_available()) {
>  		nsh = nacl_shmem();
>  		nacl_csr_write(nsh, CSR_VSSTATUS, csr->vsstatus);
> @@ -624,6 +630,7 @@ 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 +652,8 @@ 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);
> +
>  	vcpu->cpu = -1;
>  
>  	kvm_riscv_vcpu_aia_put(vcpu);
> -- 
> 2.43.0
>

This looks like a good idea, but can't we also apply it to
kvm_riscv_vcpu_aia_load()? And, if we could track whether or
not the kernel uses FP and/or vector then we could also avoid
restoring those registers when they haven't been used.

Thanks,
drew

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