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

[Guo Ren]

On Sun, May 24, 2026 at 1:23 PM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
>
> On Sat, Apr 25, 2026 at 6:29 AM <guoren@xxxxxxxxxx> wrote:
> >
> > From: "Guo Ren (Alibaba DAMO Academy)" <guoren@xxxxxxxxxx>
> >
> > 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.
Okay.

>
> >
> > This makes KVM AIA robust on big.LITTLE-style and multi-vendor
> > asymmetric platforms.
> >
> > Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@xxxxxxxxxx>
> > ---
> >  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);

Thanks for the review!

On keeping kvm_riscv_aia_nr_hgei as the minimum across all HARTs:

I agree to keep kvm_riscv_aia_nr_hgei as a non-zero indicator for HGEI
enablement. However, I have a concern about making it track the
per-HART minimum. The purpose of this patch is precisely to let each
HART use its own nr_hgei independently, so heterogeneous HARTs with
more HGEI lines are not artificially limited by weaker ones.
Collapsing that back into a global minimum would partially defeat the
goal.

ref: global->nr_guest_files = min(global->nr_guest_files,
local->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



-- 
Best Regards
 Guo Ren