Re: [PATCH RFC v4 1/4] RISC-V: paravirt: Add pvqspinlock KVM backend

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] kvm: new SBI_EXT_PVLOCK extension is enabled by default breaking migration
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commit f1e45d3725cd5aab3846041669bc52a712a0d928
Author: GUO Ren (XuanTie) <guoren@kernel.org>

RISC-V: paravirt: Add pvqspinlock KVM backend

Add the KVM backend for the SBI PVLOCK (paravirt qspinlock kick_cpu)
extension. kvm_sbi_ext_pvlock_kick_cpu() kicks the target vCPU out of
the halted state via kvm_vcpu_kick(), and yields to it with
kvm_vcpu_yield_to() when the target is already runnable.

> diff --git a/arch/riscv/kvm/vcpu_sbi_pvlock.c b/arch/riscv/kvm/vcpu_sbi_pvlock.c
> new file mode 100644
> index 0000000000000..728e59f3f18fb
> --- /dev/null
> +++ b/arch/riscv/kvm/vcpu_sbi_pvlock.c
[ ... ]
> +const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_pvlock = {
> +	.extid_start = SBI_EXT_PVLOCK,
> +	.extid_end = SBI_EXT_PVLOCK,
> +	.handler = kvm_sbi_ext_pvlock_handler,
> +};

[Severity: Critical]
Does this extension need to be disabled by default to prevent ABI breakage?

Since vcpu_sbi_ext_pvlock omits .default_disabled, it evaluates to false,
causing KVM to default the extension to KVM_RISCV_SBI_EXT_STATUS_ENABLED
during VM initialization.

Silently enabling new guest-visible KVM features breaks live migration.
Migrating the VM to an older host kernel without this feature will cause
the guest to crash when it attempts to use the extension.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260921-pvqspinlock-v4-0-409a22aed6ef@kernel.org?part=1