Re: [Xen-devel] PV guest with PCI passthrough crash on Xen 4.8.3 inside KVM when booted through OVMF

[Jason Andryuk <jandryuk@xxxxxxxxx>]

On Mon, Nov 27, 2023 at 6:27 AM Marek Marczykowski-Górecki
<marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Nov 27, 2023 at 11:20:36AM +0000, Frediano Ziglio wrote:
> > On Sun, Nov 26, 2023 at 2:51 PM Marek Marczykowski-Górecki
> > <marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Feb 19, 2018 at 06:30:14PM +0100, Juergen Gross wrote:
> > > > On 16/02/18 20:02, Andrew Cooper wrote:
> > > > > On 16/02/18 18:51, Marek Marczykowski-Górecki wrote:
> > > > >> On Fri, Feb 16, 2018 at 05:52:50PM +0000, Andrew Cooper wrote:
> > > > >>> On 16/02/18 17:48, Marek Marczykowski-Górecki wrote:
> > > > >>>> Hi,
> > > > >>>>
> > > > >>>> As in the subject, the guest crashes on boot, before kernel output
> > > > >>>> anything. I've isolated this to the conditions below:
> > > > >>>>  - PV guest have PCI device assigned (e1000e emulated by QEMU in 
> > > > >>>> this case),
> > > > >>>>    without PCI device it works
> > > > >>>>  - Xen (in KVM) is started through OVMF; with seabios it works
> > > > >>>>  - nested HVM is disabled in KVM
> > > > >>>>  - AMD IOMMU emulation is disabled in KVM; when enabled qemu 
> > > > >>>> crashes on
> > > > >>>>    boot (looks like qemu bug, unrelated to this one)
> > > > >>>>
> > > > >>>> Version info:
> > > > >>>>  - KVM host: OpenSUSE 42.3, qemu 2.9.1, 
> > > > >>>> ovmf-2017+git1492060560.b6d11d7c46-4.1, AMD
> > > > >>>>  - Xen host: Xen 4.8.3, dom0: Linux 4.14.13
> > > > >>>>  - Xen domU: Linux 4.14.13, direct boot
> > > > >>>>
> > > > >>>> Not sure if relevant, but initially I've tried booting xen.efi 
> > > > >>>> /mapbs
> > > > >>>> /noexitboot and then dom0 kernel crashed saying something about 
> > > > >>>> conflict
> > > > >>>> between e820 and kernel mapping. But now those options are 
> > > > >>>> disabled.
> > > > >>>>
> > > > >>>> The crash message:
> > > > >>>> (XEN) d1v0 Unhandled invalid opcode fault/trap [#6, ec=0000]
> > > > >>>> (XEN) domain_crash_sync called from entry.S: fault at 
> > > > >>>> ffff82d080218720 entry.o#create_bounce_frame+0x137/0x146
> > > > >>>> (XEN) Domain 1 (vcpu#0) crashed on cpu#1:
> > > > >>>> (XEN) ----[ Xen-4.8.3  x86_64  debug=n   Not tainted ]----
> > > > >>>> (XEN) CPU:    1
> > > > >>>> (XEN) RIP:    e033:[<ffffffff826d9156>]
> > > > >>> This is #UD, which is most probably hitting a BUG().  addr2line 
> > > > >>> this ^
> > > > >>> to find some code to look at.
> > > > >> addr2line failed me
> > > > >
> > > > > By default, vmlinux is stripped and compressed.  Ideally you want to
> > > > > addr2line the vmlinux artefact in the root of your kernel build, which
> > > > > is the plain elf with debugging symbols.
> > > > >
> > > > > Alternatively, use scripts/extract-vmlinux on the binary you actually
> > > > > booted, which might get you somewhere.
> > > > >
> > > > >> , but System.map says its xen_memory_setup. And it
> > > > >> looks like the BUG() is the same as I had in dom0 before:
> > > > >> "Xen hypervisor allocated kernel memory conflicts with E820 map".
> > > > >
> > > > > Juergen: Is there anything we can do to try and insert some dummy
> > > > > exception handlers right at PV start, so we could at least print out a
> > > > > oneliner to the host console which is a little more helpful than Xen
> > > > > saying "something unknown went wrong" ?
> > > >
> > > > You mean something like commit 42b3a4cb5609de757f5445fcad18945ba9239a07
> > > > added to kernel 4.15?
> > > >
> > > > >
> > > > >>
> > > > >> Disabling e820_host in guest config solved the problem. Thanks!
> > > > >>
> > > > >> Is this some bug in Xen or OVMF, or is it expected behavior and 
> > > > >> e820_host
> > > > >> should be avoided?
> > > > >
> > > > > I don't really know.  e820_host is a gross hack which shouldn't really
> > > > > be present.  The actually problem is that Linux can't cope with the
> > > > > memory layout it was given (and I can't recall if there is anything
> > > > > Linux could potentially to do cope).  OTOH, the toolstack, which knew
> > > > > about e820_host and chose to lay the guest out in an overlapping way 
> > > > > is
> > > > > probably also at fault.
> > > >
> > > > The kernel can cope with lots of E820 scenarios (e.g. by relocating
> > > > initrd or the p2m map), but moving itself out of the way is not
> > > > possible.
> > >
> > > I'm afraid I need to resurrect this thread...
> > >
> > > With recent kernel (6.6+), the host_e820=0 workaround is not an option
> > > anymore. It makes Linux not initialize xen-swiotlb (due to
> > > f9a38ea5172a3365f4594335ed5d63e15af2fd18), so PCI passthrough doesn't
> > > work at all. While I can add yet another layer of workaround (force
> > > xen-swiotlb with iommu=soft), that's getting unwieldy.
> > >
> > > Furthermore, I don't get the crash message anymore, even with debug
> > > hypervisor and guest_loglvl=all. Not even "Domain X crashed" in `xl
> > > dmesg`. It looks like the "crash" shutdown reason doesn't reach Xen, and
> > > it's considered clean shutdown (I can confirm it by changing various
> > > `on_*` settings (via libvirt) and observing which gets applied).
> > >
> > > Most tests I've done with 6.7-rc1, but the issue I observed on 6.6.1
> > > already.
> > >
> > > This is on Xen 4.17.2. And the L0 is running Linux 6.6.1, and then uses
> > > QEMU 8.1.2 + OVMF 202308 to run Xen as L1.
> > >
> >
> > So basically you start the domain and it looks like it's shutting down
> > cleanly from logs.
> > Can you see anything from the guest? Can you turn on some more
> > debugging at guest level?
>
> No, it crashes before printing anything to the console, also with
> earlyprintk=xen.
>
> > I tried to get some more information from the initial crash but I
> > could not understand which guest code triggered the bug.
>
> I'm not sure which one is it this time (because I don't have Xen
> reporting guest crash...) but last time it was here:
> https://github.com/torvalds/linux/blob/master/arch/x86/xen/setup.c#L873-L874

Hi Marek,

I too have run into this "Xen hypervisor allocated kernel memory
conflicts with E820 map" error when running Xen under KVM & OVMF with
SecureBoot.  OVMF built without SecureBoot did not trip over the
issue.  It was a little while back - I have some notes though.

Non-SecureBoot
(XEN)  [0000000000810000, 00000000008fffff] (ACPI NVS)
(XEN)  [0000000000900000, 000000007f8eefff] (usable)

SecureBoot
(XEN)  [0000000000810000, 000000000170ffff] (ACPI NVS)
(XEN)  [0000000001710000, 000000007f0edfff] (usable)

Linux (under Xen) is checking that _pa(_text) (= 0x1000000) is RAM,
but it is not.  Looking at the E820 map, there is type 4, NVS, region
defined:
[0000000000810000, 000000000170ffff] (ACPI NVS)

When OVMF is built with SMM (for SecureBoot) and S3Supported is true,
the memory range 0x900000-0x170ffff is additionally marked ACPI NVS
and Linux trips over this.  It becomes usable RAM under Non-SecureBoot
so Linux boots fine.

What I don't understand is why there is even a check that _pa(_text)
is RAM.  Xen logs that it places dom0 way up high in memory, so the
physical address of the kernel pages are much higher than 0x1000000.
The value 0x1000000 for _pa(_text) doesn't match reality.  Maybe there
are some expectations for the ACPI NVS and other reserved regions to
be 1-1 mapped?  I tried removing the BUG mentioned above, but it still
failed to boot.  I think I also removed a second BUG, but
unfortunately I don't have notes on either.

The other thing I noticed is _pa() uses phys_base to shift its output,
but phys_base is not updated under Xen.  If _pa() is supposed to
reference the high memory addresses Xen assigned, then maybe that
should be updated?

Booting under SecureBoot was tangential to my goal at the time, so I
didn't pursue it further.

Regards,
Jason