Re: [PATCH 6/6] LoongArch: KVM: Enable separate vmid feature

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] CSR_GTLBC_USEVMID is not cleared when virtualization is disabled, leaving the CPU in an inconsistent state and leaking hypervisor state to the underlying hardware.
--

--- Patch [6]: [PATCH 6/6] LoongArch: KVM: Enable separate vmid feature ---
commit 255d457e2558ae0a9e092e8d465e179d08390542
Author: Bibo Mao <maobibo@loongson.cn>

LoongArch: KVM: Enable separate vmid feature

With CSR GTLBC shortname for Guest TLB Control Register, separate vmid
feature will be enabled if bit 14 CSR_GTLBC_USEVMID is set. Enable
this feature if cpu_has_guestid is true when LVZ is enabled and the LVZ
version is 2.

> diff --git a/arch/loongarch/kvm/main.c b/arch/loongarch/kvm/main.c
> index 86b1af823e2b4..43f5b40786f24 100644
> --- a/arch/loongarch/kvm/main.c
> +++ b/arch/loongarch/kvm/main.c
> @@ -365,7 +365,7 @@ int kvm_arch_enable_virtualization_cpu(void)
>  	write_csr_gcfg(0);
>  	write_csr_gstat(0);
>  	write_csr_gintc(0);
> -	clear_csr_gtlbc(CSR_GTLBC_USETGID | CSR_GTLBC_TOTI);
> +	clear_csr_gtlbc(CSR_GTLBC_USETGID | CSR_GTLBC_TOTI | CSR_GTLBC_USEVMID);

[Severity: Medium]
Since this clears the newly added CSR_GTLBC_USEVMID bit during initialization,
should it also be cleared in kvm_arch_disable_virtualization_cpu() when
hardware virtualization is disabled?

Currently, kvm_arch_disable_virtualization_cpu() only clears the prior bits:

arch/loongarch/kvm/main.c:kvm_arch_disable_virtualization_cpu() {
	write_csr_gcfg(0);
	write_csr_gstat(0);
	write_csr_gintc(0);
	clear_csr_gtlbc(CSR_GTLBC_USETGID | CSR_GTLBC_TOTI);
	...
}

Does missing this bit during teardown leave the CPU in an inconsistent state
by retaining KVM-specific control bits in the Guest TLB Control register?

This might cause issues during CPU offline/online cycles, kexec into a
different kernel, or when another hypervisor initializes the CPU.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260727072109.2151675-1-maobibo@loongson.cn?part=6