On Tue, 13 Apr 2021, Mike Rapoport wrote:
>
> I think I've found the reason. trim_snb_memory() reserved the entire first
> megabyte very early leaving no room for real mode trampoline allocation.
> Since this reservation is needed only to make sure integrated gfx does not
> access some memory, it can be safely done after memblock allocations are
> possible.
>
> I don't know if it can be fixed on the graphics device driver side, but
> from the setup_arch() perspective I think this would be the proper fix:
>
> From c05f6046137abbcbb700571ce1ac54e7abb56a7d Mon Sep 17 00:00:00 2001
> From: Mike Rapoport <r...@linux.ibm.com>
> Date: Tue, 13 Apr 2021 21:08:39 +0300
> Subject: [PATCH] x86/setup: move trim_snb_memory() later in setup_arch to fix
> boot hangs
>
> Commit a799c2bd29d1 ("x86/setup: Consolidate early memory reservations")
> moved reservation of the memory inaccessible by Sandy Bride integrated
> graphics very early and as the result on systems with such devices the
> first 1M was reserved by trim_snb_memory() which prevented the allocation
> of the real mode trampoline and made the boot hang very early.
>
> Since the purpose of trim_snb_memory() is to prevent problematic pages ever
> reaching the graphics device, it is safe to reserve these pages after
> memblock allocations are possible.
>
> Move trim_snb_memory later in boot so that it will be called after
> reserve_real_mode() and make comments describing trim_snb_memory()
> operation more elaborate.
>
> Fixes: a799c2bd29d1 ("x86/setup: Consolidate early memory reservations")
> Reported-by: Randy Dunlap <rdun...@infradead.org>
> Signed-off-by: Mike Rapoport <r...@linux.ibm.com>
Tested-by: Hugh Dickins <hu...@google.com>
Thanks Mike and Randy. ThinkPad T420s here. I didn't notice this thread
until this morning, but had been investigating bootup panic on mmotm
yesterday. I was more fortunate than Randy, in getting some console
output which soon led to a799c2bd29d1 without bisection. Expected
to go through it line by line today, but you've saved me - thanks.
> ---
> arch/x86/kernel/setup.c | 20 +++++++++++++++-----
> 1 file changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 59e5e0903b0c..ccdcfb19df1e 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -633,11 +633,16 @@ static void __init trim_snb_memory(void)
> printk(KERN_DEBUG "reserving inaccessible SNB gfx pages\n");
>
> /*
> - * Reserve all memory below the 1 MB mark that has not
> - * already been reserved.
> + * SandyBridge integrated graphic devices have a bug that prevents
> + * them from accessing certain memory ranges, namely anything below
> + * 1M and in the pages listed in the bad_pages.
> + *
> + * To avoid these pages being ever accessed by SNB gfx device
> + * reserve all memory below the 1 MB mark and bad_pages that have
> + * not already been reserved at boot time.
> */
> memblock_reserve(0, 1<<20);
> -
> +
> for (i = 0; i < ARRAY_SIZE(bad_pages); i++) {
> if (memblock_reserve(bad_pages[i], PAGE_SIZE))
> printk(KERN_WARNING "failed to reserve 0x%08lx\n",
> @@ -746,8 +751,6 @@ static void __init early_reserve_memory(void)
>
> reserve_ibft_region();
> reserve_bios_regions();
> -
> - trim_snb_memory();
> }
>
> /*
> @@ -1083,6 +1086,13 @@ void __init setup_arch(char **cmdline_p)
>
> reserve_real_mode();
>
> + /*
> + * Reserving memory causing GPU hangs on Sandy Bridge integrated
> + * graphic devices should be done after we allocated memory under
> + * 1M for the real mode trampoline
> + */
> + trim_snb_memory();
> +
> init_mem_mapping();
>
> idt_setup_early_pf();
> --
> 2.28.0
