Re: Random crashes with i386 and efi boots

[Andy Lutomirski]

> On Sep 11, 2018, at 6:30 AM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> 
> On 09/11/2018 04:52 AM, Andy Lutomirski wrote:
>>> On Sep 10, 2018, at 2:56 PM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>>> 
>>> Hi folks,
>>> 
>>> even after commit eeb89e2bb1ac ("x86/efi: Load fixmap GDT in
>>> efi_call_phys_epilog()"), my i386/efi qemu boot tests still crash randomly
>>> (roughly 5-10% of the time). As before, I don't see much useful output in
>>> the qemu log (this time it doesn't even complain about a triple fault).
>>> 
>>> Debugging shows that the crash happens in efi_call_phys_epilog().
>>> A sample log from a crashed test run is attached below. It appears that
>>> the crash happens if there is an interrupt at a critical section of the
>>> code.
>>> 
>>> While playing with the code, I found a possible fix.
>>> 
>>> diff --git a/arch/x86/platform/efi/efi_32.c b/arch/x86/platform/efi/efi_32.c
>>> index 05ca14222463..9959657127f4 100644
>>> --- a/arch/x86/platform/efi/efi_32.c
>>>  +++ b/arch/x86/platform/efi/efi_32.c
>>> @@ -85,10 +85,9 @@ pgd_t * __init efi_call_phys_prolog(void)
>>> 
>>> void __init efi_call_phys_epilog(pgd_t *save_pgd)
>>> {
>>> +    load_fixmap_gdt(0);
>>>    load_cr3(save_pgd);
>>>    __flush_tlb_all();
>>> -
>>> -    load_fixmap_gdt(0);
>>> }
>> We have IRQs on here?  It seems plausible that weâre in a window where the EFI pgd doesnât have cpu_entry_area mapped. Also, the hard coded CPU 0 is suspicious.
> The hard coded CPU 0 was always there. The call is ultimately from
> efi_enter_virtual_mode(), which is called from start_kernel().
> so presumably it is guaranteed to run on CPU 0.
> 
>> Maybe try instrumenting the code to check whether the clone_pgd_range calls in setup_percpu.c have happened yet?
> The crash is seen late in the boot process, so I am quite sure it happened,
> but I can add a check if needed. I think that might be a different problem,
> though.
> 
>> Your patch may well be correct, but, if we have IRQs on, we should really have cpu_entry_area mapped in both pgds.
>> Or we could turn off IRQs. Why on Earth are IRQs on in a context where the fixmap gdt is unusable?
> 
> From arch/x86/platform/efi/efi.c:phys_efi_set_virtual_address_map():
> 
>    save_pgd = efi_call_phys_prolog();
>    local_irq_save(flags);
>    status = efi_call_phys(...);
>        local_irq_restore(flags);
> 
>        efi_call_phys_epilog(save_pgd);
> 
> So, yes, interrupts are very much enabled.

Does fixing that solve the problem?  It seems more robust.

> 
> I ran several additional test sequences. With above patch, no failures with
> 500 boots. Without it, failure rate (long term average) across 500 boots
> is around 10%. Another data point: Moving load_fixmap_gdt(0); after
> load_cr3(save_pgd); does not help; it has to come first.
> 
> Guenter