Re: [PATCH V2 1/4] RISC-V: KVM: AIA: Make HGEI number and management fully per-CPU

On Sat, Apr 25, 2026 at 6:29 AM <[email protected]> wrote:
>
> From: "Guo Ren (Alibaba DAMO Academy)" <[email protected]>
>
> Previously the number of Hypervisor Guest External Interrupt (HGEI)
> lines was stored in a single global variable `kvm_riscv_aia_nr_hgei`
> and assumed to be the same for all HARTs. This assumption does not
> hold on heterogeneous RISC-V SoCs where different cores may expose
> different HGEIE CSR widths.
>
> Introduce `nr_hgei` field into the per-CPU `struct aia_hgei_control`
> and probe the actual supported HGEI count for the current HART in
> `kvm_riscv_aia_enable()` using the standard RISC-V CSR probe
> technique:
>
>     csr_write(CSR_HGEIE, -1UL);
>     nr = fls_long(csr_read(CSR_HGEIE));
>     if (nr)
>         nr--;
>
> All HGEI allocation, free and disable paths (`kvm_riscv_aia_free_hgei()`,
> `kvm_riscv_aia_disable()`, etc.) now use the per-CPU value instead of
> the global one.
>
> The early global `kvm_riscv_aia_nr_hgei` is kept only for deciding
> whether SGEI interrupt registration is needed; the real per-HART
> initialization of lock and free_bitmap is moved to enable time.

We should keep the `kvm_riscv_aia_nr_hgei` to imply two things:
1) Minimum number of HGEI lines available across all HARTs
2) Non-zero kvm_riscv_aia_nr_hgei means HGEI is enabled

The PATCH2 is redundant and must be droped. I will take care
care of this at the time of merging.

>
> This makes KVM AIA robust on big.LITTLE-style and multi-vendor
> asymmetric platforms.
>
> Signed-off-by: Guo Ren (Alibaba DAMO Academy) <[email protected]>
> ---
>  arch/riscv/kvm/aia.c | 40 ++++++++++++++++++++++++----------------
>  1 file changed, 24 insertions(+), 16 deletions(-)
>
> diff --git a/arch/riscv/kvm/aia.c b/arch/riscv/kvm/aia.c
> index 5ec503288555..a23729052cfb 100644
> --- a/arch/riscv/kvm/aia.c
> +++ b/arch/riscv/kvm/aia.c
> @@ -23,6 +23,7 @@ struct aia_hgei_control {
>         raw_spinlock_t lock;
>         unsigned long free_bitmap;
>         struct kvm_vcpu *owners[BITS_PER_LONG];
> +       unsigned int nr_hgei;
>  };
>  static DEFINE_PER_CPU(struct aia_hgei_control, aia_hgei);
>  static int hgei_parent_irq;
> @@ -452,7 +453,7 @@ void kvm_riscv_aia_free_hgei(int cpu, int hgei)
>
>         raw_spin_lock_irqsave(&hgctrl->lock, flags);
>
> -       if (hgei > 0 && hgei <= kvm_riscv_aia_nr_hgei) {
> +       if (hgei > 0 && hgei <= hgctrl->nr_hgei) {
>                 if (!(hgctrl->free_bitmap & BIT(hgei))) {
>                         hgctrl->free_bitmap |= BIT(hgei);
>                         hgctrl->owners[hgei] = NULL;
> @@ -486,21 +487,8 @@ static irqreturn_t hgei_interrupt(int irq, void *dev_id)
>
>  static int aia_hgei_init(void)
>  {
> -       int cpu, rc;
> +       int rc;
>         struct irq_domain *domain;
> -       struct aia_hgei_control *hgctrl;
> -
> -       /* Initialize per-CPU guest external interrupt line management */
> -       for_each_possible_cpu(cpu) {
> -               hgctrl = per_cpu_ptr(&aia_hgei, cpu);
> -               raw_spin_lock_init(&hgctrl->lock);
> -               if (kvm_riscv_aia_nr_hgei) {
> -                       hgctrl->free_bitmap =
> -                               BIT(kvm_riscv_aia_nr_hgei + 1) - 1;
> -                       hgctrl->free_bitmap &= ~BIT(0);
> -               } else
> -                       hgctrl->free_bitmap = 0;
> -       }
>
>         /* Skip SGEI interrupt setup for zero guest external interrupts */
>         if (!kvm_riscv_aia_nr_hgei)
> @@ -545,9 +533,29 @@ static void aia_hgei_exit(void)
>
>  void kvm_riscv_aia_enable(void)
>  {
> +       struct aia_hgei_control *hgctrl;
> +
>         if (!kvm_riscv_aia_available())
>                 return;
>
> +       hgctrl = this_cpu_ptr(&aia_hgei);
> +
> +       /* Figure-out number of bits in HGEIE */
> +       csr_write(CSR_HGEIE, -1UL);
> +       hgctrl->nr_hgei = fls_long(csr_read(CSR_HGEIE));
> +       csr_write(CSR_HGEIE, 0);
> +       if (hgctrl->nr_hgei)
> +               hgctrl->nr_hgei--;

For completness of this patch, we should also have the following:

    if (gc)
        hgctrl->nr_hgei= min((ulong)hgctrl->nr_hgei, gc->nr_guest_files);
    else
        hgctrl->nr_hgei = 0;

Also, over here we should check and update kvm_riscv_aia_nr_hgei
to keep it minimum accross HARTs:

  if (hgctrl->nr_hgei && hgctrl->nr_hgei < kvm_riscv_aia_nr_hgei)
    kvm_riscv_aia_nr_hgei = hgctrl->nr_hgei;

> +
> +       if (hgctrl->nr_hgei) {
> +               hgctrl->free_bitmap = BIT(hgctrl->nr_hgei + 1) - 1;
> +               hgctrl->free_bitmap &= ~BIT(0);
> +       } else {
> +               hgctrl->free_bitmap = 0;
> +       }
> +
> +       raw_spin_lock_init(&hgctrl->lock);
> +
>         csr_write(CSR_HVICTL, aia_hvictl_value(false));
>         csr_write(CSR_HVIPRIO1, 0x0);
>         csr_write(CSR_HVIPRIO2, 0x0);
> @@ -588,7 +596,7 @@ void kvm_riscv_aia_disable(void)
>
>         raw_spin_lock_irqsave(&hgctrl->lock, flags);
>
> -       for (i = 0; i <= kvm_riscv_aia_nr_hgei; i++) {
> +       for (i = 0; i <= hgctrl->nr_hgei; i++) {
>                 vcpu = hgctrl->owners[i];
>                 if (!vcpu)
>                         continue;
> --
> 2.43.0
>

Regards,
Anup

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