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

[Werner Sembach]

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@xxxxxxxxxx>
> > > >
> > > > 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