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

[Bjorn Helgaas]

On Thu, Dec 08, 2022 at 08:16:31PM +0100, Hans de Goede wrote:
> Hi Bjorn,
> 
> On 12/8/22 19:57, Bjorn Helgaas wrote:
> > On Wed, Dec 07, 2022 at 04:31:12PM +0100, Hans de Goede wrote:
> >> On 12/4/22 10:13, Hans de Goede wrote:
> >>
> >> <snip>
> >>
> >>>>> 2. I am afraid that now allowing PCI MMIO space to be allocated
> >>>>> in regions marked as EfiMemoryMappedIO will cause regressions
> >>>>> on some systems. Specifically when I tried something similar
> >>>>> the last time I looked at this (using the BIOS date cut-off
> >>>>> approach IIRC) there was a suspend/resume regression on
> >>>>> a Lenovo ThinkPad X1 carbon (20A7) model:
> >>>>>
> >>>>> https://bugzilla.redhat.com/show_bug.cgi?id=2029207
> >>>>>
> >>>>> Back then I came to the conclusion that the problem is that not
> >>>>> avoiding the EfiMemoryMappedIO regions caused PCI MMIO space to
> >>>>> be allocated in the 0xdfa00000 - 0xdfa10000 range which is
> >>>>> listed in the EFI memmap as:
> >>>>>
> >>>>> [    0.000000] efi: mem46: [MMIO        |RUN|  |  |  |  |  |  |  |  |   |  |  |  |  ] range=[0x00000000dfa00000-0x00000000dfa0ffff] (0MB)
> >>>>>
> >>>>> And with current kernels with the extra logging added for this
> >>>>> the following is logged related to this:
> >>>>>
> >>>>> [    0.326504] acpi PNP0A08:00: clipped [mem 0xdfa00000-0xfebfffff window] to [mem 0xdfa10000-0xfebfffff window] for e820 entry [mem 0xdceff000-0xdfa0ffff]
> >>>>>
> >>>>> I believe patch 1/4 of this set will make this clipping go away,
> >>>>> re-introducing the suspend/resume problem.
> >>>>
> >>>> Yes, I'm afraid you're right.  Comparing the logs at comment #31
> >>>> (fails) and comment #38 (works):
> >>>>
> >>>>   pci_bus 0000:00: root bus resource [mem 0xdfa00000-0xfebfffff window]
> >>>>   pci 0000:00:1c.0: BAR 14: assigned [mem 0xdfa00000-0xdfbfffff] fails
> >>>>   pci 0000:00:1c.0: BAR 14: assigned [mem 0xdfb00000-0xdfcfffff] works
> >>>>
> >>>> Since 0xdfa00000 is included in the host bridge _CRS, but isn't
> >>>> usable, my guess is this is a _CRS bug.
> >>>
> >>> Ack.
> >>>
> >>> So I was thinking to maybe limit the removal of EfiMemoryMappedIO
> >>> regions from the E820 map if they are big enough to cause troubles?
> >>>
> >>> Looking at the EFI map MMIO regions on this Lenovo ThinkPad X1 carbon
> >>> (20A7) model, they are tiny. Where as the ones which we know cause
> >>> problems are huge. So maybe add a bit of heuristics to patch 1/4 based
> >>> on the EfiMemoryMappedIO region size and only remove the big ones
> >>> from the E820 map ?
> >>>
> >>> I know that adding heuristics like this always feels a bit wrong,
> >>> because you end up putting a somewhat arbitrary cut off point in
> >>> the code on which to toggle behavior on/off, but I think that in
> >>> this case it should work nicely given how huge the EfiMemoryMappedIO
> >>> regions which are actually causing problems are.
> > 
> > I'll post a v2 that removes only regions 256KB or larger in a minute.
> 
> Ok, may I ask why 256KB?
> 
> I see that that rules out then troublesome MMIO regions from the X1 carbon from:
> https://bugzilla.redhat.com/show_bug.cgi?id=2029207 :
> efi: mem46: [MMIO|RUN|  ] range=[0xdfa00000-0xdfa0ffff] (0MB) [64K]
> which we know we need to avoid / keep reserved.
> 
> But OTOH the reservations which are causing the problems with assigning
> resources to PCI devices by Linux look like this:
> efi: mem50: [MMIO        |RUN|  |  |  |  |  |  |  |  |   |  |  |  |UC] range=[0x0000000065400000-0x00000000cfffffff] (1708MB)
> which is significantly larger then 256KB.
> 
> So we could e.g. also put the cut-off point at 16MB and still
> remove the above troublesome reservation from the E820 table.
> Note just thinking out loud here. I have no idea if 16MB
> would be better...

No good reason for 256KB.  We know it needs to be at least 64KB for
the X1 Carbon.  I picked 4x bigger just for headroom, since I assume
the 64KB is platform-specific host bridge registers or something.  Do
you think a bigger number would be better, i.e., we would retain more
MMIO things in E820?

ECAM areas would be 1MB per bus, so between 1MB and 256MB.  Those areas
*should* be reserved by PNP0C02 _CRS, but IIRC the early MMCONFIG code
checks E820, and the late code checks for _CRS.  I guess one could
argue that ignoring those, e.g., by retaining anything 256MB or
smaller in E820, would reduce the amount of change.  

But if the host bridge _CRS includes 256MB of legitimate window that
EFI says is MMIO and is hence included in E820, that seems like kind
of a lot of usable window space to give up.

> ...
> Sorry for the confusion. What I was trying to say is that I was interested
> in seeing if we could use the "RUN" flag to differentiate between:
> 
> 1. The big MMIO region which we want to remove from the e820 map:
>    efi: mem50: [MMIO        |RUN|  |  |  |  |  |  |  |  |   |  |  |  |UC] range=[0x0000000065400000-0x00000000cfffffff] (1708MB)
> 
> 2. The small MMIO region which we want to keep to avoid the reported suspend/resume issue:
>    efi: mem46: [MMIO|RUN|  ] range=[0xdfa00000-0xdfa0ffff] (0MB) [64K]
> 
> But unfortunately both have the RUN flag set so the RUN flag is
> of no use to us.

Right, makes sense.

Bjorn