Re: Unable to start VM with 5.10-rc3

Ben Gardon
2020-11-11T01:13:37Z

Hi Zdenek,

That crash is most likely the result of a missing check for an invalid
root HPA or NULL shadow page in is_tdp_mmu_root, which could have
prevented the NULL pointer dereference.
However, I'm not sure how a vCPU got to that point in the page fault
handler with a bad EPT root page.

I see VMX in your list of flags, is your machine 64 bit with EPT or
some other configuration?

I'm surprised you are finding your machine unable to boot for
bisecting. Do you know if it's crashing in the same spot or somewhere
else? I wouldn't expect the KVM page fault handler to run as part of
boot.

I will send out a patch first thing tomorrow morning (PST) to WARN
instead of crashing with a NULL pointer dereference. Are you able to
reproduce the issue with any KVM selftest?

Ben

On Tue, Nov 10, 2020 at 7:24 AM Zdenek Kaspar <zkaspar82@gmail.com> wrote:
>
> Hi,
>
> attached file is result from today's linux-master (with fixes
> for 5.10-rc4) when I try to start VM on older machine:
>
> model name : Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz
> flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ht tm pbe syscall nx lm constant_tsc arch_perfmon pebs bts rep_good nopl cpuid aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm lahf_lm pti tpr_shadow dtherm
> vmx flags : tsc_offset vtpr
>
> I did quick check with 5.9 (distro kernel) and it works,
> but VM performance seems extremely impacted. 5.8 works fine.
>
> Back to 5.10 issue: it's problematic since 5.10-rc1 and I have no luck
> with bisecting (machine doesn't boot).
>
> TIA, Z.