Re: [PATCH v4] RISC-V: KVM: Serialize virtual interrupt pending state updates

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Restoring guest `sip` (or `siph`) CSR via KVM_SET_ONE_REG drops pending interrupts from KVM's internal `irqs_pending` tracker, causing a vCPU stall if the guest executes WFI.
--

--- Patch [1]: [PATCH v4] RISC-V: KVM: Serialize virtual interrupt pending state updates ---
commit 445393d6bd22386cd7da7af1c9dd4c9d3be703a4
Author: Xie Bo <xb@ultrarisc.com>

RISC-V: KVM: Serialize virtual interrupt pending state updates

RISC-V KVM tracks guest interrupt state with two bitmaps: irqs_pending and
irqs_pending_mask. This commit serializes all updates to these bitmaps
with a per-vCPU raw spinlock to fix a race condition where injected
interrupts could be lost.

> diff --git a/arch/riscv/kvm/vcpu_onereg.c b/arch/riscv/kvm/vcpu_onereg.c
> index bb920e8923c93..cba3682944b69 100644
> --- a/arch/riscv/kvm/vcpu_onereg.c
> +++ b/arch/riscv/kvm/vcpu_onereg.c
[ ... ]
> @@ -309,10 +310,14 @@ static int kvm_riscv_vcpu_general_set_csr(struct kvm_vcpu *vcpu,
>  		reg_val <<= VSIP_TO_HVIP_SHIFT;
>  	}
>  
> -	((unsigned long *)csr)[reg_num] = reg_val;
> -
> -	if (reg_num == KVM_REG_RISCV_CSR_REG(sip))
> -		WRITE_ONCE(vcpu->arch.irqs_pending_mask[0], 0);
> +	if (reg_num == KVM_REG_RISCV_CSR_REG(sip)) {
> +		raw_spin_lock_irqsave(&vcpu->arch.irqs_pending_lock, flags);
> +		((unsigned long *)csr)[reg_num] = reg_val;
> +		vcpu->arch.irqs_pending_mask[0] = 0;

[Severity: High]
This is a pre-existing issue, but does restoring the guest sip (or siph)
CSR via KVM_SET_ONE_REG drop pending interrupts from KVM's internal
irqs_pending tracker?

If userspace (e.g., QEMU) migrating a VM writes to KVM_REG_RISCV_CSR_REG(sip),
csr->hvip is updated and irqs_pending_mask[0] is cleared. However,
irqs_pending[0] is not updated with the restored state.

Later, if the guest executes WFI with interrupts locally disabled but
globally expected to wake it, kvm_riscv_vcpu_has_interrupts() will return
false because it solely checks irqs_pending:

arch/riscv/kvm/vcpu.c:kvm_riscv_vcpu_has_interrupts() {
    ...
	ret = vcpu->arch.irqs_pending[0] & ie;
    ...
}

Could this cause a VM hang or stalled vCPU after live migration if it enters
WFI?

> +		raw_spin_unlock_irqrestore(&vcpu->arch.irqs_pending_lock, flags);
> +	} else {
> +		((unsigned long *)csr)[reg_num] = reg_val;
> +	}
>  
>  	return 0;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260713073346.1293408-2-xb@ultrarisc.com?part=1