On 28 January 2022 21:40:42 GMT, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>On Fri, Jan 28, 2022, David Woodhouse wrote:
>> On Fri, 2021-12-17 at 14:55 -0600, Tom Lendacky wrote:
>> > On 12/17/21 2:13 PM, David Woodhouse wrote:
>> > > On Fri, 2021-12-17 at 13:46 -0600, Tom Lendacky wrote:
>> > > > There's no WARN or PANIC, just a reset. I can look to try and capture some
>> > > > KVM trace data if that would help. If so, let me know what events you'd
>> > > > like captured.
>> > >
>> > >
>> > > Could start with just kvm_run_exit?
>> > >
>> > > Reason 8 would be KVM_EXIT_SHUTDOWN and would potentially indicate a
>> > > triple fault.
>> >
>> > qemu-system-x86-24093 [005] ..... 1601.759486: kvm_exit: vcpu 112 reason shutdown rip 0xffffffff81070574 info1 0x0000000000000000 info2 0x0000000000000000 intr_info 0x80000b08 error_code 0x00000000
>> >
>> > # addr2line -e woodhouse-build-x86_64/vmlinux 0xffffffff81070574
>> > /root/kernels/woodhouse-build-x86_64/./arch/x86/include/asm/desc.h:272
>> >
>> > Which is: asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
>>
>> So, I remain utterly bemused by this, and the Milan *guests* I have
>> access to can't even kexec with a stock kernel; that is also "too fast"
>> and they take a triple fault during the bringup in much the same way —
>> even without my parallel patches, and even going back to fairly old
>> kernels.
>>
>> I wasn't able to follow up with raw serial output during the bringup to
>> pinpoint precisely where it happens, because the VM would tear itself
>> down in response to the triple fault without actually flushing the last
>> virtual serial output :)
>>
>> It would be really useful to get access to a suitable host where I can
>> spawn this in qemu and watch it fail. I am suspecting a chip-specific
>> quirk or bug at this point.
>
>Nope. You missed a spot. This also reproduces on a sufficiently large Intel
>system (and Milan). initial_gs gets overwritten by common_cpu_up(), which leads
>to a CPU getting the wrong MSR_GS_BASE and then the wrong raw_smp_processor_id(),
>resulting in cpu_init_exception_handling() stuffing the wrong GDT and leaving a
>NULL TR descriptor for itself.
>
>You also have a lurking bug in the x2APIC ID handling. Stripping the boot flags
>from the prescribed APICID needs to happen before retrieving the x2APIC ID from
>CPUID, otherwise bits 31:16 of the ID will be lost.
>
>You owe me two beers ;-)
Oh Sean, I love you.
Thanks.
Will update and retest and resend.
