Hi Zdenek, I'm working on reproducing the issue. I don't have access to a CPU without EPT, but I tried turning off EPT on a Skylake and I think I reproduced the issue, but wasn't able to confirm in the logs. If you were operating without EPT I assume the guest was in non-paging mode to get into direct_page_fault in the first place. I would still have expected the root HPA to be valid unless... Ah, if you're operating with PAE, then the root hpa will be valid but not have a shadow page associated with it, as it is set to __pa(vcpu->arch.mmu->pae_root) in mmu_alloc_direct_roots. In that case, I can see why we get a NULL pointer dereference in is_tdp_mmu_root. I will send out a patch that should fix this if the issue is as described above. I don't have hardware to test this on, but if you don't mind applying the patch and checking it, that would be awesome. Ben On Wed, Nov 11, 2020 at 3:09 AM Zdenek Kaspar <zkaspar82@xxxxxxxxx> wrote: > > Hi, I'm sure my bisect has nothing to do with KVM, > because it was quick shot between -rc1 and previous release. > > This old CPU doesn't have EPT (see attached file) > > ./run_tests.sh > FAIL apic-split (timeout; duration=90s) > FAIL ioapic-split (timeout; duration=90s) > FAIL apic (timeout; duration=30) > ... ^C > few RIP is_tdp_mmu_root observed in dmesg > > Z. > > On Tue, 10 Nov 2020 17:13:21 -0800 > Ben Gardon <bgardon@xxxxxxxxxx> wrote: > > > 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@xxxxxxxxx> > > 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. >
