Re: [PATCH v2 1/1] RISCV: KVM: update external interrupt atomically for IMSIC swfile

[Anup Patel]

On Wed, Dec 13, 2023 at 11:46 AM Yong-Xuan Wang
<yongxuan.wang@xxxxxxxxxx> wrote:
>
> The emulated IMSIC update the external interrupt pending depending on the
> value of eidelivery and topei. It might lose an interrupt when it is
> interrupted before setting the new value to the pending status.
>
> For example, when VCPU0 sends an IPI to VCPU1 via IMSIC:
>
> VCPU0                           VCPU1
>
>                                 CSRSWAP topei = 0
>                                 The VCPU1 has claimed all the external
>                                 interrupt in its interrupt handler.
>
>                                 topei of VCPU1's IMSIC = 0
>
> set pending in VCPU1's IMSIC
>
> topei of VCPU1' IMSIC = 1
>
> set the external interrupt
> pending of VCPU1
>
>                                 clear the external interrupt pending
>                                 of VCPU1
>
> When the VCPU1 switches back to VS mode, it exits the interrupt handler
> because the result of CSRSWAP topei is 0. If there are no other external
> interrupts injected into the VCPU1's IMSIC, VCPU1 will never know this
> pending interrupt unless it initiative read the topei.
>
> If the interruption occurs between updating interrupt pending in IMSIC
> and updating external interrupt pending of VCPU, it will not cause a
> problem. Suppose that the VCPU1 clears the IPI pending in IMSIC right
> after VCPU0 sets the pending, the external interrupt pending of VCPU1
> will not be set because the topei is 0. But when the VCPU1 goes back to
> VS mode, the pending IPI will be reported by the CSRSWAP topei, it will
> not lose this interrupt.
>
> So we only need to make the external interrupt updating procedure as a
> critical section to avoid the problem.
>
> Fixes: db8b7e97d613 ("RISC-V: KVM: Add in-kernel virtualization of AIA IMSIC")
> Tested-by: Roy Lin <roy.lin@xxxxxxxxxx>
> Tested-by: Wayling Chen <wayling.chen@xxxxxxxxxx>
> Co-developed-by: Vincent Chen <vincent.chen@xxxxxxxxxx>
> Signed-off-by: Vincent Chen <vincent.chen@xxxxxxxxxx>
> Signed-off-by: Yong-Xuan Wang <yongxuan.wang@xxxxxxxxxx>

Queued as fixes for Linux-6.7

Thanks,
Anup

> ---
> Changelog
> v2:
> - rename the variable and add a short comment in the code
> ---
>  arch/riscv/kvm/aia_imsic.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
> index 6cf23b8adb71..e808723a85f1 100644
> --- a/arch/riscv/kvm/aia_imsic.c
> +++ b/arch/riscv/kvm/aia_imsic.c
> @@ -55,6 +55,7 @@ struct imsic {
>         /* IMSIC SW-file */
>         struct imsic_mrif *swfile;
>         phys_addr_t swfile_pa;
> +       spinlock_t swfile_extirq_lock;
>  };
>
>  #define imsic_vs_csr_read(__c)                 \
> @@ -613,12 +614,23 @@ static void imsic_swfile_extirq_update(struct kvm_vcpu *vcpu)
>  {
>         struct imsic *imsic = vcpu->arch.aia_context.imsic_state;
>         struct imsic_mrif *mrif = imsic->swfile;
> +       unsigned long flags;
> +
> +       /*
> +        * The critical section is necessary during external interrupt
> +        * updates to avoid the risk of losing interrupts due to potential
> +        * interruptions between reading topei and updating pending status.
> +        */
> +
> +       spin_lock_irqsave(&imsic->swfile_extirq_lock, flags);
>
>         if (imsic_mrif_atomic_read(mrif, &mrif->eidelivery) &&
>             imsic_mrif_topei(mrif, imsic->nr_eix, imsic->nr_msis))
>                 kvm_riscv_vcpu_set_interrupt(vcpu, IRQ_VS_EXT);
>         else
>                 kvm_riscv_vcpu_unset_interrupt(vcpu, IRQ_VS_EXT);
> +
> +       spin_unlock_irqrestore(&imsic->swfile_extirq_lock, flags);
>  }
>
>  static void imsic_swfile_read(struct kvm_vcpu *vcpu, bool clear,
> @@ -1039,6 +1051,7 @@ int kvm_riscv_vcpu_aia_imsic_init(struct kvm_vcpu *vcpu)
>         }
>         imsic->swfile = page_to_virt(swfile_page);
>         imsic->swfile_pa = page_to_phys(swfile_page);
> +       spin_lock_init(&imsic->swfile_extirq_lock);
>
>         /* Setup IO device */
>         kvm_iodevice_init(&imsic->iodev, &imsic_iodoev_ops);
> --
> 2.17.1
>