From: Mark Kettenis <mark.kettenis@xs4all.nl>
Subject: Re: Use acpipci in VMs (was: PCI BAR mapping in qemu VMs)
To: Stefan Fritsch <sf@openbsd.org>
Cc: tech@openbsd.org
Date: Mon, 31 Mar 2025 11:36:36 +0200

> Date: Fri, 21 Mar 2025 14:24:08 +0100 (CET)
> From: Stefan Fritsch <sf@openbsd.org>

Hi Stefan,

> Hi,
> 
> On Sun, 16 Mar 2025, Stefan Fritsch wrote:
> > there were some reports that vio on KVM/qemu sometimes panics with
> > 
> >  vq_size not power of two: 65535
> > 
> > but I could never reproduce it. bluhm@ now got me a test setup where the 
> > bsd kernel is PXE booted on qemu in 440fx mode, and there it is 
> > reproducible.
> > 
> > After some debugging it seems that seabios or ipxe maps the PCI BARs at 
> > 0x380000000000-0x380080000000 which is outside the allowed range in 
> > pci_init_extents(). On the other hand, in 440fx mode, qemu seems to 
> > produce ACPI 1.x tables and there is a check in acpipci_attach() that for 
> > ACPI < 5.x, the PCI infos from _CRS are not used. OpenBSD will then 
> > disable the BARs and when mapping them again in vio_attach(), it will 
> > sometimes choose adresses that do not work, reads return 0xff and writes 
> > are ignored. I guess this is becuase the address (in my case 0xbff14000) 
> > lies outside the PCI window of the emulated chipset.
> > 
> > I have put dmesg, acpi tables and other info at 
> > https://www.sfritsch.de/~stf/vq-panic/
> > 
> > Qemu in q35 mode produces ACPI 3.x tables, so it may also be affected.
> > 
> > There may be three ways to fix this:
> > 
> > 1) increase the allowed range for pcimem in pci_init_extents(). This is 
> > what the diff below does.
> > 
> > 2) somehow make acpipci_attach() use the ACPI infos on qemu. I have 
> > verified that removing the version check fixes the issue. Since removing 
> > the version check seems to break many other systems, this would have to be 
> > a qemu specific quirk.
> > 
> > 3) try to make OpenBSD reliably map the BARs somewhere where it works. Is 
> > there a way for OpenBSD to get the info where the PCI window is without 
> > trusting ACPI?
> > 
> > I remember at least one report of this issue on i386. Any idea how to fix 
> > it there?
> 
> Mark Patruck noticed that these issues seem to be caused by some 
> relatively recent changes in seabios.
> 
> https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/message/R7FOQMMYWVX577QNIA2AKUAGOZKNJIAP/
> https://gitlab.com/qemu-project/seabios/-/commit/df9dd418b3b0e586cb208125094620fc7f90f23d
> 
> A workaround seems to be to configure the VM with <= 3GB memory.
> 
> The problem may become more wide-spread with 7.7, since we now default to 
> virtio 1.x, which uses MMIO on qemu, compared to virtio 0.9 which uses PIO 
> BARs. Therefore it would be nice to get a fix in before the release, if it 
> is not too late already.
> 
> The diff below uses acpipci / _CRS also with old ACPI versions if running 
> on a hypervisor. I think the chance that it will break unrelated systems 
> is low. It does not change behavior on vmd, where no acpi attaches at all.
> 
> ok?

I hate these VM quirks.  Why are folks still emulating hardware from
the 1990's when running a modern OS?

Anyway, not much we can do about that I guess.  But maybe we can have
a bit more consistency?  We already have a check to enable MSI for
QEMU.  And since this really is a QEMU issue, maybe it would be better
to use a PCI_SUBSYS_ID_REG check here?

> diff --git a/sys/arch/amd64/pci/acpipci.c b/sys/arch/amd64/pci/acpipci.c
> index 51cd1360383..2e3236772bb 100644
> --- a/sys/arch/amd64/pci/acpipci.c
> +++ b/sys/arch/amd64/pci/acpipci.c
> @@ -152,7 +152,7 @@ acpipci_attach(struct device *parent, struct device *self, void *aux)
>  
>  	aml_parse_resource(&res, acpipci_parse_resources, sc);
>  
> -	if (sc->sc_acpi->sc_major < 5) {
> +	if (sc->sc_acpi->sc_major < 5 && (cpu_ecxfeature & CPUIDECX_HV) == 0) {
>  		extent_destroy(sc->sc_ioex);
>  		extent_destroy(sc->sc_memex);
>  
> 
>