Re: [BISECTED] v4.15-rc: Boot regression on x86_64/AMD

[Christian KÃnig]

This is a multi-part message in MIME format.
Am 09.01.2018 um 00:23 schrieb Bjorn Helgaas:
> [+cc Boris, Juergen, linux-pci]
>
> On Fri, Jan 5, 2018 at 6:00 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Fri, Jan 5, 2018 at 2:04 PM, Aaro Koskinen <aaro.koskinen@xxxxxx> wrote:
> > > After v4.14, I've been unable to boot my AMD compilation box with the
> > > v4.15-rc mainline Linux. It just ends up in a silent reboot loop.
> > >
> > > I bisected this to:
> > >
> > > commit fa564ad9636651fd11ec2c79c48dee844066f73a
> > > Author: Christian KÃnig <christian.koenig@xxxxxxx>
> > > Date:   Tue Oct 24 14:40:29 2017 -0500
> > >
> > >      x86/PCI: Enable a 64bit BAR on AMD Family 15h (Models 00-1f, 30-3f, 60-7f)
> > Hmm. That was reported to break boot earlier already.
> >
> > The breakage was supposedly fixed by three patches from Christian:
> >
> >    a19e2696135e: "x86/PCI: Only enable a 64bit BAR on single-socket AMD
> > Family 15h"
> >
> >    470195f82e4e:  "x86/PCI: Fix infinite loop in search for 64bit BAR placement"
> >
> > and a third one that was apparently never applied.
> >
> > I'm not sure why that third patch was never applied, I'm including it here.
> >
> > Does the system work for you if you apply that patch (instead of
> > reverting all of them)?
> >
> > I wonder why that patch wasn't applied, but if it doesn't fix things,
> > I think we do need to revert it all.
> >
> > Christian? Bjorn?
> I didn't apply the third patch ("x86/PCI: limit the size of the 64bit
> BAR to 256GB") because (a) we thought it was optional ("just a
> precaution against eventual problems"), (b) we didn't have a good
> explanation of why 256GB was the correct number, and (c)  it seemed to
> be a workaround for a Xen issue that we hoped to fix in a better way.

Just for the record completely agree on that.

> It does apparently make Aaro's system work, but I still hesitate to
> apply it because it's magical -- avoiding the address space from
> 0x1_00000000 to 0xbd_00000000 makes things work, but we don't know
> why.  I assume there's some unreported device in that area, but I
> don't think we have any real assurance that the
> 0xbd_00000000-0xfd_00000000 area we now use is any safer.

Well, I knew why it's not working. The BIOS is not telling us the truth 
about how much memory is installed.

A device above 4GB would actually be handled correctly by the code (see 
the check when we walk over all the existing IO regions).

I tested a bit with Aaro and came up with the attached patch, it adds a 
16GB guard between the end of memory and the new window for the PCIe 
root hub. But I agree with you that this is just a hack and not a real 
solution.

> I would feel better about this if we made it opt-in via a kernel
> parameter and/or some kind of whitelist.  I still don't really *like*
> it, since ACPI does provide a mechanism (_PRS/_SRS) for doing this
> safely, and we could just say "if you want to use big BARs, the BIOS
> should enable big windows or at least make them available via ACPI
> resources."  The only problem is that BIOSes don't do that and we
> don't yet have Linux support for _PRS/_SRS for host bridges.

Well that is the point I disagree on. When the memory map we get from 
the BIOS is not correct it makes no difference if we enable the window 
with the BIOS or by direct programming the hardware.

I will work with Aaron some more to come up with a solution which reads 
the memory map directly from the hardware as well and checks if that is 
valid before doing anything else.

> I'll prepare a revert as a back-up plan in case we don't come up with
> a better solution.

Either that or only enable it when pci=add-root-window is given on the 
kernel commandline.

Just let me know what you prefer and I will hack a patch for this 
together today.

Christian.

> Bjorn