Re: [PATCH v3 1/2] RISC-V: KVM: Separate req and fallback_req masks in make_xfence_request

On Fri, Jul 31, 2026 at 3:12 PM Wang Yechao <[email protected]> wrote:
>
> When handling hfence requests in make_xfence_request(), the current code
> uses a single 'actual_req' variable and a single vcpu_mask. If any VCPU
> fails to enqueue the hfence data (because its queue is full), the request
> falls back to 'fallback_req' for all VCPUs, even if other VCPUs still
> have available queue space.
>
> This can cause unnecessary fallback for healthy VCPUs, and more seriously,
> those healthy VCPUs will not process their already-enqueued hfence
> requests because no 'req' is set for them. As a result, their queues will
> quickly become full as well, degrading performance for SMP guests.
>
> Fix this by maintaining two separate bitmaps: one for VCPUs that
> successfully enqueued the hfence data (req_vcpu_mask) and another for
> those that failed (fallback_req_vcpu_mask). Then send the appropriate
> requests to each group. This ensures that fallback is only applied to
> VCPUs that actually need it, preserving the efficiency of the normal
> path for others.
>
> Fixes: 13acfec2dbcc ("RISC-V: KVM: Add remote HFENCE functions based on VCPU requests")
> Signed-off-by: Wang Yechao <[email protected]>

LGTM.

Reviewed-by: Anup Patel <[email protected]>

Thanks,
Anup

> ---
>  arch/riscv/kvm/tlb.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/arch/riscv/kvm/tlb.c b/arch/riscv/kvm/tlb.c
> index 993b25ea94d67..c54522decaa64 100644
> --- a/arch/riscv/kvm/tlb.c
> +++ b/arch/riscv/kvm/tlb.c
> @@ -332,10 +332,11 @@ static void make_xfence_request(struct kvm *kvm,
>  {
>         unsigned long i;
>         struct kvm_vcpu *vcpu;
> -       unsigned int actual_req = req;
> -       DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
> +       DECLARE_BITMAP(req_vcpu_mask, KVM_MAX_VCPUS);
> +       DECLARE_BITMAP(fallback_req_vcpu_mask, KVM_MAX_VCPUS);
>
> -       bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
> +       bitmap_zero(req_vcpu_mask, KVM_MAX_VCPUS);
> +       bitmap_zero(fallback_req_vcpu_mask, KVM_MAX_VCPUS);
>         kvm_for_each_vcpu(i, vcpu, kvm) {
>                 if (hbase != -1UL) {
>                         if (vcpu->vcpu_id < hbase ||
> @@ -345,10 +346,10 @@ static void make_xfence_request(struct kvm *kvm,
>                                 continue;
>                 }
>
> -               bitmap_set(vcpu_mask, i, 1);
> -
> -               if (!data || !data->type)
> +               if (!data || !data->type) {
> +                       bitmap_set(req_vcpu_mask, i, 1);
>                         continue;
> +               }
>
>                 /*
>                  * Enqueue hfence data to VCPU hfence queue. If we don't
> @@ -356,10 +357,13 @@ static void make_xfence_request(struct kvm *kvm,
>                  * a more conservative hfence request.
>                  */
>                 if (!vcpu_hfence_enqueue(vcpu, data))
> -                       actual_req = fallback_req;
> +                       bitmap_set(fallback_req_vcpu_mask, i, 1);
> +               else
> +                       bitmap_set(req_vcpu_mask, i, 1);
>         }
>
> -       kvm_make_vcpus_request_mask(kvm, actual_req, vcpu_mask);
> +       kvm_make_vcpus_request_mask(kvm, req, req_vcpu_mask);
> +       kvm_make_vcpus_request_mask(kvm, fallback_req, fallback_req_vcpu_mask);
>  }
>
>  void kvm_riscv_fence_i(struct kvm *kvm,
> --
> 2.39.3
>