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Tsirkin" , qemu-devel@nongnu.org, Stefano Stabellini , Anthony Perard , Paul Durrant , Paolo Bonzini , Richard Henderson , Eduardo Habkost , Marcel Apfelbaum , xen-devel@lists.xenproject.org Subject: Re: [PATCH v6] xen/pt: reserve PCI slot 2 for Intel igd-passthru Message-ID: <20230103081456.1d676b8e.alex.williamson@redhat.com> In-Reply-To: References: <830263507e8f1a24a94f81909d5102c4b204e938.1672615492.git.brchuckz.ref@aol.com> <830263507e8f1a24a94f81909d5102c4b204e938.1672615492.git.brchuckz@aol.com> <20230102124605-mutt-send-email-mst@kernel.org> Organization: Red Hat MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 2 Jan 2023 18:10:24 -0500 Chuck Zmudzinski wrote: > On 1/2/23 12:46 PM, Michael S. Tsirkin wrote: > > On Sun, Jan 01, 2023 at 06:52:03PM -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 the old and no longer maintained Qemu traditional device model available 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. > > > > > > The new xen_igd_reserve_slot function also needs to be implemented in > > > hw/xen/xen_pt_stub.c to prevent FTBFS during the link stage for the case > > > when Qemu is configured with --enable-xen and --disable-xen-pci-passthrough, > > > in which case it does nothing. > > > > > > The new xen_igd_clear_slot function overrides qdev->realize of the parent > > > PCI device class to enable the Intel IGD to occupy slot 2 on the PCI bus > > > since slot 2 was reserved by xen_igd_reserve_slot when the PCI bus was > > > created in hw/i386/pc_piix.c for the case when igd-passthru=on. > > > > > > Move the call to xen_host_pci_device_get, and the associated error > > > handling, from xen_pt_realize to the new xen_igd_clear_slot function to > > > initialize the device class and vendor values which enables the checks for > > > the Intel IGD to succeed. The verification that the host device is an > > > Intel IGD to be passed through is done by checking the domain, bus, slot, > > > and function values as well as by checking that gfx_passthru is enabled, > > > the device class is VGA, and the device vendor in Intel. > > > > > > Signed-off-by: Chuck Zmudzinski > > > > I'm not sure why is the issue xen specific. Can you explain? > > Doesn't it affect kvm too? > > Recall from docs/igd-assign.txt that there are two modes for > igd passthrough: legacy and upt, and the igd needs to be > at slot 2 only when using legacy mode which gives one > single guest exclusive access to the Intel igd. > > It's only xen specific insofar as xen does not have support > for the upt mode so xen must use legacy mode which > requires the igd to be at slot 2. I am not an expert with UPT mode never fully materialized for direct assignment, the folks at Intel championing this scenario left. > kvm, but if I understand correctly, with kvm one can use > the upt mode with the Intel i915 kvmgt kernel module > and in that case the guest will see a virtual Intel gpu > that can be at any arbitrary slot when using kvmgt, and > also, in that case, more than one guest can access the > igd through the kvmgt kernel module. This is true, IIRC an Intel vGPU does not need to be in slot 2. > Again, I am not an expert and do not have as much > experience with kvm, but if I understand correctly it is > possible to use the legacy mode with kvm and I think you > are correct that if one uses kvm in legacy mode and without > using the Intel i915 kvmgt kernel module, then it would be > necessary to reserve slot 2 for the igd on kvm. It's necessary to configure the assigned IGD at slot 2 to make it functional, yes, but I don't really understand this notion of "reserving" slot 2. If something occupies address 00:02.0 in the config, it's the user's or management tool's responsibility to move it to make this configuration functional. Why does QEMU need to play a part in reserving this bus address. IGD devices are not generally hot-pluggable either, so it doesn't seem we need to reserve an address in case an IGD device is added dynamically later. > Your question makes me curious, and I have not been able > to determine if anyone has tried igd passthrough using > legacy mode on kvm with recent versions of linux and qemu. Yes, it works. > I will try reproducing the problem on kvm in legacy mode with > current versions of linux and qemu and report my findings. > With kvm, there might be enough flexibility to specify the > slot number for every pci device in the guest. Such a I think this is always the recommendation, libvirt will do this by default in order to make sure the configuration is reproducible. This is what we generally rely on for kvm/vfio IGD assignment to place the GPU at the correct address. > capability is not available using the xenlight toolstack > for managing xen guests, so I have been using this patch > to ensure that the Intel igd is at slot 2 with xen guests > created by the xenlight toolstack. Seems like a deficiency in xenlight. I'm not sure why QEMU should take on this burden to support support tool stacks that lack such basic features. > The patch as is will only fix the problem on xen, so if the > problem exists on kvm also, I agree that the patch should > be modified to also fix it on kvm. AFAICT, it's not a problem on kvm/vfio because we generally make use of invocations that specify bus addresses for each device by default, making this a configuration requirement for the user or management tool stack. Thanks, Alex