[RFC] LoongArch: KVM: VCPU_EVENTS for pending interrupt/exception migration

[RFC] LoongArch: KVM: VCPU_EVENTS for pending interrupt/exception migration

[Tao Cui]

Hi Bibo,

I'd like to get your feedback on an approach for migrating the pending
interrupt and exception state (irq_pending, irq_clear, exception_pending,
esubcode) that is currently not captured by the CSR-based migration path.

I noticed your earlier work on this in the "Small enhancement about
interrupt injection" series — specifically the kvm_vcpu_sync_intr() patch
([PATCH v3 6/6], 2026-05-19), which tried to fold irq_pending into the
software ESTAT register before migration reads.  I see it was removed in
v4 with the note that it "does not sync cached" state correctly, and the
rest of the series was merged without it.

The core difficulty, as I understand it, is that irq_pending represents
queued-but-not-yet-delivered interrupts, while ESTAT represents the
already-delivered state — folding the former into the latter would make
the guest see interrupts it should not yet see.

My alternative approach is to add KVM_CAP_VCPU_EVENTS with a
LoongArch-specific struct that captures these fields independently,
without modifying ESTAT:

  struct kvm_vcpu_events {
      __u64 irq_pending;
      __u64 irq_clear;
      __u64 exception_pending;
      __u32 esubcode;
      __u32 reserved[11];
  };

Userspace (QEMU) calls KVM_GET_VCPU_EVENTS during migration save and
KVM_SET_VCPU_EVENTS during restore, keeping the queued/delivered
distinction intact.  The kernel handler is straightforward — just
read/write the four fields to/from vcpu->arch.

I've implemented both sides and verified end-to-end on a 3A6000:
- Kernel: 3-file patch (uapi struct + cap advertisement + GET/SET
  ioctl handlers), built on linux-next 20260805.
- QEMU: 4-file patch (LA kvm header struct + CPUState fields +
  VMState subsection + get/put in save/load path), built on
  QEMU 11.1-rc3.
- strace on a real source→dest migration confirms QEMU probes
  KVM_CAP_VCPU_EVENTS (=1), then issues KVM_GET_VCPU_EVENTS on save
  and KVM_SET_VCPU_EVENTS on restore for both vCPUs (all return 0).

Do you think this approach is reasonable?  I wanted to check with you
before submitting, since you've worked in this area and might have
insights on why the fold-into-ESTAT path was preferred (or if there
are concerns with the separate-capture approach I'm missing).

Thanks,
Tao

Re: [RFC] LoongArch: KVM: VCPU_EVENTS for pending interrupt/exception migration

[Bibo Mao]

Hi Tao,

On 2026/8/6 下午4:04, Tao Cui wrote:
> Hi Bibo,
> 
> I'd like to get your feedback on an approach for migrating the pending
> interrupt and exception state (irq_pending, irq_clear, exception_pending,
> esubcode) that is currently not captured by the CSR-based migration path.
> 
> I noticed your earlier work on this in the "Small enhancement about
> interrupt injection" series — specifically the kvm_vcpu_sync_intr() patch
> ([PATCH v3 6/6], 2026-05-19), which tried to fold irq_pending into the
> software ESTAT register before migration reads.  I see it was removed in
> v4 with the note that it "does not sync cached" state correctly, and the
> rest of the series was merged without it.
> 
> The core difficulty, as I understand it, is that irq_pending represents
> queued-but-not-yet-delivered interrupts, while ESTAT represents the
The main problem is that dmsintc is added recently, AI agent reports 
that interrupt status of dmsintc should be synced also, and synced to 
software state LOONGARCH_CSR_ISR0-LOONGARCH_CSR_ISR3 register from local 
state ds->vector_map[], similiar with function dmsintc_inject_irq().

If dmsintc is synced to software state, hardware registers about 
CSR_ISR0-CSR_ISR3 will be stale. And it need to restore from software 
status in _kvm_vcpu_load() even if KVM_LARCH_HWCSR_USABLE in 
vcpu->arch.aux_inuse is set.

That will be bigger change, and also AI agent reports variable vector[] 
is accessed without initialized issue. So this patch is removed :( 
However I think that vcpu_load()/vcpu_put() can be removed in 
_kvm_getcsr() at final.

> already-delivered state — folding the former into the latter would make
> the guest see interrupts it should not yet see.
> 
> My alternative approach is to add KVM_CAP_VCPU_EVENTS with a
> LoongArch-specific struct that captures these fields independently,
> without modifying ESTAT:
> 
>    struct kvm_vcpu_events {
>        __u64 irq_pending;
>        __u64 irq_clear;
>        __u64 exception_pending;
>        __u32 esubcode;
>        __u32 reserved[11];
>    };
Adding new uapi will lead to compatible issue. I think it is not necessary.

Regards
Bibo Mao
> 
> Userspace (QEMU) calls KVM_GET_VCPU_EVENTS during migration save and
> KVM_SET_VCPU_EVENTS during restore, keeping the queued/delivered
> distinction intact.  The kernel handler is straightforward — just
> read/write the four fields to/from vcpu->arch.
> 
> I've implemented both sides and verified end-to-end on a 3A6000:
> - Kernel: 3-file patch (uapi struct + cap advertisement + GET/SET
>    ioctl handlers), built on linux-next 20260805.
> - QEMU: 4-file patch (LA kvm header struct + CPUState fields +
>    VMState subsection + get/put in save/load path), built on
>    QEMU 11.1-rc3.
> - strace on a real source→dest migration confirms QEMU probes
>    KVM_CAP_VCPU_EVENTS (=1), then issues KVM_GET_VCPU_EVENTS on save
>    and KVM_SET_VCPU_EVENTS on restore for both vCPUs (all return 0).
> 
> Do you think this approach is reasonable?  I wanted to check with you
> before submitting, since you've worked in this area and might have
> insights on why the fold-into-ESTAT path was preferred (or if there
> are concerns with the separate-capture approach I'm missing).
> 
> Thanks,
> Tao
> 

Re: [RFC] LoongArch: KVM: VCPU_EVENTS for pending interrupt/exception migration

[Tao Cui]

在 2026/8/6 16:48, Bibo Mao 写道:
> Hi Tao,
> 
> On 2026/8/6 下午4:04, Tao Cui wrote:
>> Hi Bibo,
>>
>> I'd like to get your feedback on an approach for migrating the pending
>> interrupt and exception state (irq_pending, irq_clear, exception_pending,
>> esubcode) that is currently not captured by the CSR-based migration path.
>>
>> I noticed your earlier work on this in the "Small enhancement about
>> interrupt injection" series — specifically the kvm_vcpu_sync_intr() patch
>> ([PATCH v3 6/6], 2026-05-19), which tried to fold irq_pending into the
>> software ESTAT register before migration reads.  I see it was removed in
>> v4 with the note that it "does not sync cached" state correctly, and the
>> rest of the series was merged without it.
>>
>> The core difficulty, as I understand it, is that irq_pending represents
>> queued-but-not-yet-delivered interrupts, while ESTAT represents the
> The main problem is that dmsintc is added recently, AI agent reports that interrupt status of dmsintc should be synced also, and synced to software state LOONGARCH_CSR_ISR0-LOONGARCH_CSR_ISR3 register from local state ds->vector_map[], similiar with function dmsintc_inject_irq().
> 
> If dmsintc is synced to software state, hardware registers about CSR_ISR0-CSR_ISR3 will be stale. And it need to restore from software status in _kvm_vcpu_load() even if KVM_LARCH_HWCSR_USABLE in vcpu->arch.aux_inuse is set.
> 
> That will be bigger change, and also AI agent reports variable vector[] is accessed without initialized issue. So this patch is removed :( However I think that vcpu_load()/vcpu_put() can be removed in _kvm_getcsr() at final.
Thanks for the explanation, the DMSINTC part makes sense now.
> 
>> already-delivered state — folding the former into the latter would make
>> the guest see interrupts it should not yet see.
>>
>> My alternative approach is to add KVM_CAP_VCPU_EVENTS with a
>> LoongArch-specific struct that captures these fields independently,
>> without modifying ESTAT:
>>
>>    struct kvm_vcpu_events {
>>        __u64 irq_pending;
>>        __u64 irq_clear;
>>        __u64 exception_pending;
>>        __u32 esubcode;
>>        __u32 reserved[11];
>>    };
> Adding new uapi will lead to compatible issue. I think it is not necessary.
> 
I'll drop the VCPU_EVENTS idea.

Since you're already working on this direction, I'll wait for your
patch.  Happy to help test locally once it's ready.

Thanks,
Tao
> Regards
> Bibo Mao
>>
>> Userspace (QEMU) calls KVM_GET_VCPU_EVENTS during migration save and
>> KVM_SET_VCPU_EVENTS during restore, keeping the queued/delivered
>> distinction intact.  The kernel handler is straightforward — just
>> read/write the four fields to/from vcpu->arch.
>>
>> I've implemented both sides and verified end-to-end on a 3A6000:
>> - Kernel: 3-file patch (uapi struct + cap advertisement + GET/SET
>>    ioctl handlers), built on linux-next 20260805.
>> - QEMU: 4-file patch (LA kvm header struct + CPUState fields +
>>    VMState subsection + get/put in save/load path), built on
>>    QEMU 11.1-rc3.
>> - strace on a real source→dest migration confirms QEMU probes
>>    KVM_CAP_VCPU_EVENTS (=1), then issues KVM_GET_VCPU_EVENTS on save
>>    and KVM_SET_VCPU_EVENTS on restore for both vCPUs (all return 0).
>>
>> Do you think this approach is reasonable?  I wanted to check with you
>> before submitting, since you've worked in this area and might have
>> insights on why the fold-into-ESTAT path was preferred (or if there
>> are concerns with the separate-capture approach I'm missing).
>>
>> Thanks,
>> Tao
>>
>