Re: [PATCH 0/4] PCI: Continue E820 vs host bridge window saga

[Werner Sembach <wse@tuxedocomputers.com>]

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)
> 
> Note I know you did not propose to drop the quirks in this series,
> just covering all the bases here.
> 
> Regards,
> 
> Hans
> 
> 
> 
> 
Kind regards,
Werner Sembach