Re: [PATCH v7] xen/pt: reserve PCI slot 2 for Intel igd-passthru

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jan 09, 2023 at 06:28:44PM -0500, Chuck Zmudzinski wrote:
> On 1/9/23 5:33 PM, Michael S. Tsirkin wrote:
> > On Mon, Jan 09, 2023 at 04:55:42PM -0500, Chuck Zmudzinski wrote:
> >> Intel specifies that the Intel IGD must occupy slot 2 on the PCI bus,
> >> as noted in docs/igd-assign.txt in the Qemu source code.
> >> 
> >> Currently, when the xl toolstack is used to configure a Xen HVM guest with
> >> Intel IGD passthrough to the guest with the Qemu upstream device model,
> >> a Qemu emulated PCI device will occupy slot 2 and the Intel IGD will occupy
> >> a different slot. This problem often prevents the guest from booting.
> >> 
> >> The only available workaround is not good: Configure Xen HVM guests to use
> >> the old and no longer maintained Qemu traditional device model available
> >> from xenbits.xen.org which does reserve slot 2 for the Intel IGD.
> >> 
> >> To implement this feature in the Qemu upstream device model for Xen HVM
> >> guests, introduce the following new functions, types, and macros:
> >> 
> >> * XEN_PT_DEVICE_CLASS declaration, based on the existing TYPE_XEN_PT_DEVICE
> >> * XEN_PT_DEVICE_GET_CLASS macro helper function for XEN_PT_DEVICE_CLASS
> >> * typedef XenPTQdevRealize function pointer
> >> * XEN_PCI_IGD_SLOT_MASK, the value of slot_reserved_mask to reserve slot 2
> >> * xen_igd_reserve_slot and xen_igd_clear_slot functions
> >> 
> >> The new xen_igd_reserve_slot function uses the existing slot_reserved_mask
> >> member of PCIBus to reserve PCI slot 2 for Xen HVM guests configured using
> >> the xl toolstack with the gfx_passthru option enabled, which sets the
> >> igd-passthru=on option to Qemu for the Xen HVM machine type.
> > 
> > I don't like how slot_reserved_mask is set initially then cleared on
> > device realize.
> > To me this looks like a fragile hack. I suggest one of the following
> > 1. adding a new mask
> > "slot-manual-mask" or some such blocking auto-allocation of a given
> > slot without blocking its use if address is specified on command line.
> > 2. adding a special property that overrides slot_reserved_mask
> > for a given device.
> > 
> > both need changes in pci core but look like something generally
> > useful.
> 
> I was hoping to not need to touch pci core but I understand it would be
> better for this patch to not affect machines that are manually configured
> on the command line.
> 
> However, keep in mind that this patch will only actually reserve the slot
> initially for xen hvm machines (machine type "xenfv") that also are configured
> with the qemu igd-passthru=on option which, AFAIK, only applies to machines
> witn accel=xen. It will not affect kvm users at all. So I don't think this patch
> will break many machines out there that manually specify the pci slots. The
> only machines it could affect are machines configured for igd-passthru on xen.
> This patch also does *not* reserve the slot initially for "xenfv" machines that
> are not configured with igd passthrough which I am sure is the vast majority
> of all the xen virtual machines out in the wild.

I'm just saying that adding a capability that is generally useful
as opposed to xen specific means less technical debt.

-- 
MST