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SubjectRe: [PATCH 0/4] PCI: Continue E820 vs host bridge window saga
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Hi Werner,

On 12/5/22 14:27, Werner Sembach wrote:
> Hi
>
> Am 04.12.22 um 10:29 schrieb Hans de Goede:
>> Hi Bjorn,
>>
>> On 12/3/22 18:57, Bjorn Helgaas wrote:
>>> On Sat, Dec 03, 2022 at 01:44:10PM +0100, Hans de Goede wrote:
>>>> Hi Bjorn,
>>>>
>>>> On 12/2/22 22:18, Bjorn Helgaas wrote:
>>>>> From: Bjorn Helgaas <bhelgaas@google.com>
>>>>>
>>>>> When allocating space for PCI BARs, Linux avoids allocating space mentioned
>>>>> in the E820 map.  This was originally done by 4dc2287c1805 ("x86: avoid
>>>>> E820 regions when allocating address space") to work around BIOS defects
>>>>> that included unusable space in host bridge _CRS.
>>>>>
>>>>> Some recent machines use EfiMemoryMappedIO for PCI MMCONFIG and host bridge
>>>>> apertures, and bootloaders and EFI stubs convert those to E820 regions,
>>>>> which means we can't allocate space for hot-added PCI devices (often a
>>>>> dock) or for devices the BIOS didn't configure (often a touchpad)
>>>>>
>>>>> The current strategy is to add DMI quirks that disable the E820 filtering
>>>>> on these machines and to disable it entirely starting with 2023 BIOSes:
>>>>>
>>>>>    d341838d776a ("x86/PCI: Disable E820 reserved region clipping via quirks")
>>>>>    0ae084d5a674 ("x86/PCI: Disable E820 reserved region clipping starting in 2023")
>>>>>
>>>>> But the quirks are problematic because it's really hard to list all the
>>>>> machines that need them.
>>>>>
>>>>> This series is an attempt at a more generic approach.  I'm told by firmware
>>>>> folks that EfiMemoryMappedIO means "the OS should map this area so EFI
>>>>> runtime services can use it in virtual mode," but does not prevent the OS
>>>>> from using it.
>>>>>
>>>>> The first patch removes any EfiMemoryMappedIO areas from the E820 map.
>>>>> This doesn't affect any virtual mapping of those areas (that would have to
>>>>> be done directly from the EFI memory map) but it means Linux can allocate
>>>>> space for PCI MMIO.
>>>>>
>>>>> The rest are basically cosmetic log message changes.
>>>>
>>>> Thank you for working on this. I'm a bit worried about this series though.
>>>>
>>>> The 2 things which I worry about are:
>>>>
>>>>
>>>> 1. I think this will not help when people boot in BIOS (CSM) mode rather
>>>> then UEFI mode which quite a few Linux users still do because they learned
>>>> to do this years ago when Linux EFI support (and EFI fw itself) was still
>>>> a bit in flux.
>>>>
>>>> IIRC from the last time we looked at this in CSM mode the BIOS itself
>>>> translates the EfiMemoryMappedIO areas to reserved E820 regions. So when
>>>> people use the BIOS CSM mode to boot, then this patch will not help
>>>> since the kernel lacks the info to do the translation.
>>>
>>> Right, if BIOS CSM puts EfiMemoryMappedIO in the E820 map the same way
>>> bootloaders do, and the kernel doesn't have the EFI memory map, this
>>> series won't help.
>>
>> So I just got the requested dmesg in BIOS CSM mode from:
>> https://bugzilla.redhat.com/show_bug.cgi?id=1868899
>>
>> And it says:
>>
>> [    0.000000] BIOS-e820: [mem 0x000000004bc50000-0x00000000cfffffff] reserved
>> [    0.316140] pci_bus 0000:00: root bus resource [mem 0x65400000-0xbfffffff window]
>>
>> So I'm afraid that I remembered correctly and the CSM adds
>> the EfiMemoryMappedIO regions to the E820 map as reserved :(
>>
>> So as you said, this series won't help for people booting in
>> BIOS compatibility mode. Which means that we should at least keep
>> the current list of no_e820 quirks to avoid regressing those models
>> when booted in BIOS compatibility mode.
>>
>> And maybe still add at least the Clevo model for which I recently
>> submitted a new no_e820 quirk so that that will work in BIOS CSM
>> mode too ?
> Do you mean the X170KM-G? I don't think it has the option to switch to Legacy BIOS mode (At least i didn't found an option in the bios version i have)

I'm talking about this patch:

https://lore.kernel.org/linux-pci/20221010150206.142615-1-hdegoede@redhat.com/

Regards,

Hans

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