linux-kernel - Re: BGRT Pointer in System RAM

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Date:	Mon, 15 Jul 2013 16:08:13 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Parag Warudkar <parag.lkml@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-acpi@...r.kernel.org
Subject: Re: BGRT Pointer in System RAM

On Mon, Jul 15, 2013 at 4:04 PM, Josh Triplett <josh@...htriplett.org> wrote:
> On Mon, Jul 15, 2013 at 01:28:36PM -0700, Andy Lutomirski wrote:
>> On Sun, Jul 14, 2013 at 12:07 PM, Parag Warudkar <parag.lkml@...il.com> wrote:
>> > Saw this warning running latest git (Ubuntu daily mainline.) It looked
>> > similar to what Andy saw on MSI hardware -
>> > http://www.spinics.net/lists/linux-acpi/msg43410.html . The patch for
>> > it doesn't seem to be merged, although it won't help in my case -
>> > different hardware with valid status instead of invalid and image
>> > address falling in system RAM instead of just being wild.
>> >
>> > Unsure  how this should be handled - moving the is_ram() check in
>> > efi_bgrt_init and ignoring the BGRT in case where the check succeeds?
>> > Doesn't sound completely right to me - since the BGRT is valid and
>> > exists somewhere, but..
>> >
>> > [    0.015141] ------------[ cut here ]------------
>> > [    0.015147] WARNING: CPU: 0 PID: 0 at
>> > /home/apw/COD/linux/arch/x86/mm/ioremap.c:102
>> > __ioremap_caller+0x312/0x390()
>> > [ snip ]
>> > [    0.015160] Call Trace:
>> > [    0.015165]  [<ffffffff8170a704>] dump_stack+0x46/0x58
>> > [    0.015169]  [<ffffffff8106406c>] warn_slowpath_common+0x8c/0xc0
>> > [    0.015171]  [<ffffffff810640ba>] warn_slowpath_null+0x1a/0x20
>> > [    0.015173]  [<ffffffff81054e32>] __ioremap_caller+0x312/0x390
>> > [    0.015176]  [<ffffffff814013d4>] ? acpi_tb_verify_table+0x54/0x58
>> > [    0.015179]  [<ffffffff81d35551>] ? efi_bgrt_init+0x8f/0x143
>> > [    0.015181]  [<ffffffff81054f07>] ioremap_nocache+0x17/0x20
>> > [    0.015183]  [<ffffffff81d35551>] efi_bgrt_init+0x8f/0x143
>> > [    0.015186]  [<ffffffff81401d36>] ? acpi_tb_initialize_facs+0x32/0x34
>> > [    0.015188]  [<ffffffff81d34e7f>] efi_late_init+0x9/0xb
>> > [    0.015190]  [<ffffffff81d18f17>] start_kernel+0x3fd/0x419
>> > [    0.015192]  [<ffffffff81d189ac>] ? do_early_param+0x87/0x87
>> > [    0.015194]  [<ffffffff81d18120>] ? early_idt_handlers+0x120/0x120
>> > [    0.015196]  [<ffffffff81d185e6>] x86_64_start_reservations+0x2a/0x2c
>> >
>> > ioremap.c:102
>> >     /*
>> >          * Don't allow anybody to remap normal RAM that we're using..
>> >          */
>> >         last_pfn = last_addr >> PAGE_SHIFT;
>> >         for (pfn = phys_addr >> PAGE_SHIFT; pfn <= last_pfn; pfn++) {
>> >                 int is_ram = page_is_ram(pfn);
>> >
>> >                 if (is_ram && pfn_valid(pfn) && !PageReserved(pfn_to_page(pfn)))
>> >                         return NULL;
>> >                 WARN_ON_ONCE(is_ram);
>> >         }
>> >
>> > Looking at the BGRT table from IASL, the status seems to be valid but
>> > the image address *seems* to me that is falling under system RAM.
>>
>> Interesting.  My BGRT says:
>>
>> [028h 0040   8]                Image Address : 0D06801800000001
>>
>> If I reverse the high and low 32-bit dwords, then I get an address in
>> system RAM.
>
> Does that address in RAM start with a BMP header?

No idea.  I'd presumably have to modify the driver to find out --
otherwise something else will overwrite it.

>
> Because that would be *special*.  I don't think it's worth trying to
> cope with that bug; better to just write off the BGRT as invalid if the
> BIOS can't get endianness right.
>
> In theory we could guess at that bug if the unmangled address points to
> a location in RAM starting with a BMP header.  In practice, let's not; a
> missing BGRT is a purely cosmetic issue, and BIOS vendors can learn to
> get that one right if they want to see their logo during Linux boot.
> This won't break fastboot support, it just breaks fancy crossfades from
> the BIOS logo to a Linux desktop or splash.

FWIW, the address that my BIOS gives is non-canonical.  Maybe that's
good enough.

>
> So, a "firmware bug" message in dmesg seems sufficent for that case.  We
> do need to handle the case of a valid pointer into memory that e820
> calls system RAM, as well as the case of a valid pointer into memory
> reserved for the BIOS or similar, but not the case of an invalid
> pointer.

Is the efi_bgrt code called early enough that data in system RAM will
still be there?

--Andy
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