Re: Random crashes with i386 and efi boots

From: Guenter Roeck
Date: Tue Sep 11 2018 - 09:30:30 EST

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

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_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,

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();
status = efi_call_phys(...);


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.