Re: Random crashes with i386 and efi boots

[Guenter Roeck]

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.

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