hw/riscv/virt-acpi-build: Fix RINTC PLIC context ID for KVM

[PATCH] hw/riscv/virt-acpi-build: Fix RINTC PLIC context ID for KVM

Posted by Qingwei Hu 1 month ago

Each RISC-V MADT RINTC entry contains an External Interrupt Controller
ID field. On the virt machine without AIA, this field identifies the
S-mode PLIC context associated with the hart.

TCG virt has both M-mode and S-mode PLIC contexts, so the S-mode context
ID is odd and 2 * local_cpu_id + 1 is correct. KVM virt exposes only
S-mode PLIC contexts, and those contexts are numbered contiguously from
0. Reporting the TCG context ID for KVM makes the guest enable a
different PLIC context from the one used by QEMU.

With ACPI enabled, this can leave PCI INTx interrupts pending in QEMU
while the guest-programmed PLIC context remains disabled. A virtio-blk
root disk can then stall during boot because its first interrupt is never
delivered.

Use local_cpu_id for KVM and keep the existing odd S-mode context ID for
TCG.

Fixes: d641da6ed43 ("hw/riscv/virt-acpi-build.c: Add PLIC in MADT")
Signed-off-by: Qingwei Hu <qingwei.hu@bytedance.com>
---
 hw/riscv/virt-acpi-build.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/hw/riscv/virt-acpi-build.c b/hw/riscv/virt-acpi-build.c
index 413d47d70e..6b06d9aa87 100644
--- a/hw/riscv/virt-acpi-build.c
+++ b/hw/riscv/virt-acpi-build.c
@@ -100,6 +100,8 @@ static void riscv_acpi_madt_add_rintc(uint32_t uid,
         build_append_int_noprefix(entry,
                                   ACPI_BUILD_INTC_ID(
                                       arch_ids->cpus[uid].props.node_id,
+                                      kvm_enabled() ?
+                                      local_cpu_id :
                                       2 * local_cpu_id + 1),
                                   4);
     } else {
-- 
2.39.5

Re: [PATCH] hw/riscv/virt-acpi-build: Fix RINTC PLIC context ID for KVM

Posted by Alistair Francis 2 weeks, 4 days ago

On Thu, Jun 4, 2026 at 10:44 PM Qingwei Hu <qingwei.hu@bytedance.com> wrote:
>
> Each RISC-V MADT RINTC entry contains an External Interrupt Controller
> ID field. On the virt machine without AIA, this field identifies the
> S-mode PLIC context associated with the hart.
>
> TCG virt has both M-mode and S-mode PLIC contexts, so the S-mode context
> ID is odd and 2 * local_cpu_id + 1 is correct. KVM virt exposes only
> S-mode PLIC contexts, and those contexts are numbered contiguously from
> 0. Reporting the TCG context ID for KVM makes the guest enable a
> different PLIC context from the one used by QEMU.
>
> With ACPI enabled, this can leave PCI INTx interrupts pending in QEMU
> while the guest-programmed PLIC context remains disabled. A virtio-blk
> root disk can then stall during boot because its first interrupt is never
> delivered.
>
> Use local_cpu_id for KVM and keep the existing odd S-mode context ID for
> TCG.
>
> Fixes: d641da6ed43 ("hw/riscv/virt-acpi-build.c: Add PLIC in MADT")
> Signed-off-by: Qingwei Hu <qingwei.hu@bytedance.com>

Thanks!

Applied to riscv-to-apply.next

Alistair

> ---
>  hw/riscv/virt-acpi-build.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/hw/riscv/virt-acpi-build.c b/hw/riscv/virt-acpi-build.c
> index 413d47d70e..6b06d9aa87 100644
> --- a/hw/riscv/virt-acpi-build.c
> +++ b/hw/riscv/virt-acpi-build.c
> @@ -100,6 +100,8 @@ static void riscv_acpi_madt_add_rintc(uint32_t uid,
>          build_append_int_noprefix(entry,
>                                    ACPI_BUILD_INTC_ID(
>                                        arch_ids->cpus[uid].props.node_id,
> +                                      kvm_enabled() ?
> +                                      local_cpu_id :
>                                        2 * local_cpu_id + 1),
>                                    4);
>      } else {
> --
> 2.39.5
>
>

Re: [PATCH] hw/riscv/virt-acpi-build: Fix RINTC PLIC context ID for KVM

Posted by Nutty.Liu 1 month ago

On 6/4/2026 8:00 PM, Qingwei Hu wrote:
> Each RISC-V MADT RINTC entry contains an External Interrupt Controller
> ID field. On the virt machine without AIA, this field identifies the
> S-mode PLIC context associated with the hart.
>
> TCG virt has both M-mode and S-mode PLIC contexts, so the S-mode context
> ID is odd and 2 * local_cpu_id + 1 is correct. KVM virt exposes only
> S-mode PLIC contexts, and those contexts are numbered contiguously from
> 0. Reporting the TCG context ID for KVM makes the guest enable a
> different PLIC context from the one used by QEMU.
>
> With ACPI enabled, this can leave PCI INTx interrupts pending in QEMU
> while the guest-programmed PLIC context remains disabled. A virtio-blk
> root disk can then stall during boot because its first interrupt is never
> delivered.
>
> Use local_cpu_id for KVM and keep the existing odd S-mode context ID for
> TCG.
>
> Fixes: d641da6ed43 ("hw/riscv/virt-acpi-build.c: Add PLIC in MADT")
> Signed-off-by: Qingwei Hu <qingwei.hu@bytedance.com>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>

Thanks,
Nutty
> ---
>   hw/riscv/virt-acpi-build.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/hw/riscv/virt-acpi-build.c b/hw/riscv/virt-acpi-build.c
> index 413d47d70e..6b06d9aa87 100644
> --- a/hw/riscv/virt-acpi-build.c
> +++ b/hw/riscv/virt-acpi-build.c
> @@ -100,6 +100,8 @@ static void riscv_acpi_madt_add_rintc(uint32_t uid,
>           build_append_int_noprefix(entry,
>                                     ACPI_BUILD_INTC_ID(
>                                         arch_ids->cpus[uid].props.node_id,
> +                                      kvm_enabled() ?
> +                                      local_cpu_id :
>                                         2 * local_cpu_id + 1),
>                                     4);
>       } else {

Re: [PATCH] hw/riscv/virt-acpi-build: Fix RINTC PLIC context ID for KVM

Posted by Sunil V L 1 month ago

On Thu, Jun 4, 2026 at 6:14 PM Qingwei Hu <qingwei.hu@bytedance.com> wrote:
>
> Each RISC-V MADT RINTC entry contains an External Interrupt Controller
> ID field. On the virt machine without AIA, this field identifies the
> S-mode PLIC context associated with the hart.
>
> TCG virt has both M-mode and S-mode PLIC contexts, so the S-mode context
> ID is odd and 2 * local_cpu_id + 1 is correct. KVM virt exposes only
> S-mode PLIC contexts, and those contexts are numbered contiguously from
> 0. Reporting the TCG context ID for KVM makes the guest enable a
> different PLIC context from the one used by QEMU.
>
> With ACPI enabled, this can leave PCI INTx interrupts pending in QEMU
> while the guest-programmed PLIC context remains disabled. A virtio-blk
> root disk can then stall during boot because its first interrupt is never
> delivered.
>
> Use local_cpu_id for KVM and keep the existing odd S-mode context ID for
> TCG.
>
> Fixes: d641da6ed43 ("hw/riscv/virt-acpi-build.c: Add PLIC in MADT")
> Signed-off-by: Qingwei Hu <qingwei.hu@bytedance.com>
> ---
>  hw/riscv/virt-acpi-build.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/hw/riscv/virt-acpi-build.c b/hw/riscv/virt-acpi-build.c
> index 413d47d70e..6b06d9aa87 100644
> --- a/hw/riscv/virt-acpi-build.c
> +++ b/hw/riscv/virt-acpi-build.c
> @@ -100,6 +100,8 @@ static void riscv_acpi_madt_add_rintc(uint32_t uid,
>          build_append_int_noprefix(entry,
>                                    ACPI_BUILD_INTC_ID(
>                                        arch_ids->cpus[uid].props.node_id,
> +                                      kvm_enabled() ?
> +                                      local_cpu_id :
>                                        2 * local_cpu_id + 1),
>                                    4);
>      } else {
> --
>
LGTM.

Reviewed-by: Sunil V L <sunilvl@oss.qualcomm.com>