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SubjectRe: [PATCH] x86/PCI: never allocate PCI space from the last 1M below 4G
On 11/29/2010 12:34 PM, Bjorn Helgaas wrote:
> On Monday, November 29, 2010 11:30:09 am Bjorn Helgaas wrote:
>>
>> The last 1M before 4G contains the processor restart vector and usually
>> the system ROM. We don't know the actual ROM size; I chose 1M because
>> that's how much Windows 7 appears to avoid.
>>
>> Without this check, we can allocate PCI space that will never work. On
>> Matthew's HP 2530p, we put the Intel GTT "Flush Page" at the very last
>> page, which causes a spontaneous power-off:
>>
>> pci_root PNP0A08:00: host bridge window [mem 0xfee01000-0xffffffff]
>> fffff000-ffffffff : Intel Flush Page (assigned by intel-gtt)
>>
>> Reference: https://bugzilla.kernel.org/show_bug.cgi?id=23542
>> Reported-by: Matthew Garrett <mjg@redhat.com>
>> Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
>> ---
>>
>> arch/x86/include/asm/e820.h | 3 +++
>> arch/x86/pci/i386.c | 10 +++++++++-
>> 2 files changed, 12 insertions(+), 1 deletions(-)
>>
>>
>> diff --git a/arch/x86/include/asm/e820.h b/arch/x86/include/asm/e820.h
>> index 5be1542..c1e908f 100644
>> --- a/arch/x86/include/asm/e820.h
>> +++ b/arch/x86/include/asm/e820.h
>> @@ -72,6 +72,9 @@ struct e820map {
>> #define BIOS_BEGIN 0x000a0000
>> #define BIOS_END 0x00100000
>>
>> +#define BIOS_ROM_BASE 0xfff00000
>> +#define BIOS_ROM_END 0x100000000ULL
>
> I'm really not thrilled about hard-coding these addresses, so I'd
> love it if somebody could suggest a way to discover them from the
> BIOS.
>
> The E820 map doesn't reserve the last page:
>
> BIOS-e820: 00000000fed1c000 - 00000000fed20000 (reserved)
> BIOS-e820: 00000000fffa0000 - 00000000fffa7000 (reserved)
>
> and I don't think there's any ACPI device that does either.
>

It is certainly reasonable to block off the last chunk of the 32-bit
address space. Some systems double-decode it to avoid issues with
A20M#, so I would argue that we should avoid at least 2 MiB.

As far as discovering them from the BIOS, there is a way to do it --
E820. This is a fallback for the case where the BIOS has plain and
simply failed to provide it, and so a heuristic is probably the best we
can do. Probing is extremely unsafe.

-hpa


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