Re: [PATCH 1/2] LoongArch: KVM: Fix PC double advance in kernel MMIO read fast path

[Tao Cui]

Hi, Kaiyang

在 2026/9/2 18:12, Kaiyang Wu 写道:
> On 2026-07-23 19:15, Zeng Chi wrote:
> 
>> In the in-kernel MMIO read fast path of kvm_emu_mmio_read(),
>> kvm_complete_mmio_read() already advances the guest PC via update_pc().
>> The explicit update_pc() call right after it advances the PC a second
>> time, so PC moves forward by 8 instead of 4 and the instruction
>> following the MMIO read is silently skipped.
>>
>> The user space MMIO read completion path in kvm_arch_vcpu_ioctl_run()
>> calls kvm_complete_mmio_read() only once, and the MMIO write fast path
>> advances the PC exactly once as well.
>>
>> Remove the redundant update_pc() so the kernel MMIO read fast path
>> advances the PC by a single instruction.
> 
> Hi Zeng,
> 
> I have tested the patch on Proxmox VE with the environment below:
> 
> - CPU: Loongson 3C6000/D
> - OS: Proxmox VE 9.2.11 (LoongArch port https://github.com/pve-loong64-port)
> - Kernel: 7.0.14-15-pve
> - Guest OS: Kylin Linux Advanced Server V10 (SP3 2403)
> - Guest Kernel: 4.19.90-89.45.v2401.a.ky10.loongarch64
> 
> Without the patch, the guest system could not boot with the 4.19.90-89.45.v2401.a.ky10.loongarch64 kernel and there was no output in neither tty nor serial. I can confirm the system boots from the same kernel version with the patch applied to the host kernel.
> 

Thank you for the testing, this is really helpful.

Our test setup is a bit different: we were running a 7.x kernel on
both the host and the guest, and we confirmed the issue by observing
that the PC advanced by the wrong step after an MMIO read.

Your result is a nice complement to ours — it suggests the same issue
may also explain the boot failure of a 4.19 guest on a 7.x host. We
had been debugging this boot failure for a while without finding the
root cause, and had assumed it was some kind of version
incompatibility. Your testing gives us a good lead to look into.

Thanks again for sharing your results.

--
Tao> Best,
> Kaiyang