On Fri, Jul 25, 2025 at 02:53:37PM +0100, Jonathan Cameron wrote:
> On Fri, 18 Jul 2025 21:35:45 +0800
> peng guo <engguop...@buaa.edu.cn> wrote:
>
> > When using a CXL Type 3 device together with a virtio 9p device in QEMU, the
> > 9p device fails to initialize properly. The kernel reports the following:
> >
> > virtio: device uses modern interface but does not have
> > VIRTIO_F_VERSION_1
> > 9pnet_virtio virtio0: probe with driver 9pnet_virtio failed with error
> > -22
> >
> > Further investigation revealed that the 64-bit BAR space assigned to the
> > 9pnet
> > device was overlapped by the memory window allocated for the CXL devices.
> > As a
> > result, the kernel could not correctly access the BAR region, causing the
> > virtio device to malfunction.
> >
> > An excerpt from /proc/iomem shows:
> >
> > 480010000-cffffffff : CXL Window 0
> > 480010000-4bfffffff : PCI Bus 0000:00
> > 4c0000000-4c01fffff : PCI Bus 0000:0c
> > 4c0000000-4c01fffff : PCI Bus 0000:0d
> > 4c0200000-cffffffff : PCI Bus 0000:00
> > 4c0200000-4c0203fff : 0000:00:03.0
> > 4c0200000-4c0203fff : virtio-pci-modern
> >
> > To address this issue, this patch uses the value of `cxl_resv_end` to
> > reserve
> > sufficient address space and ensure that CXL memory windows are allocated
> > beyond all PCI 64-bit BARs. This prevents overlap with 64-bit BARs regions
> > such
> > as those used by virtio or other pcie devices, resolving the conflict.
> >
> > QEMU Build Configuration:
> >
> > ./configure --prefix=/home/work/qemu_master/build/ \
> > --target-list=x86_64-softmmu \
> > --enable-kvm \
> > --enable-virtfs
> >
> > QEMU Boot Command:
> >
> > sudo /home/work/qemu_master/qemu/build/qemu-system-x86_64 \
> > -nographic -machine q35,cxl=on -enable-kvm -m 16G -smp 8 \
> > -hda /home/work/gp_qemu/rootfs.img \
> > -virtfs
> > local,path=/home/work/gp_qemu/share,mount_tag=host0,security_model=passthrough,id=host0
> > \
> > -kernel /home/work/linux_output/arch/x86/boot/bzImage \
> > --append "console=ttyS0 crashkernel=256M root=/dev/sda
> > rootfstype=ext4 rw loglevel=8" \
> > -object memory-backend-ram,id=vmem0,share=on,size=4096M \
> > -device pxb-cxl,bus_nr=12,bus=pcie.0,id=cxl.1 \
> > -device cxl-rp,port=0,bus=cxl.1,id=root_port13,chassis=0,slot=2 \
> > -device
> > cxl-type3,bus=root_port13,volatile-memdev=vmem0,id=cxl-vmem0,sn=0x123456789
> > \
> > -M cxl-fmw.0.targets.0=cxl.1,cxl-fmw.0.size=4G
> >
> > Tested in a QEMU setup with a CXL Type 3 device and a 9pnet virtio device.
> >
> > Signed-off-by: peng guo <engguop...@buaa.edu.cn>
> Analysis looks good.
>
> For the patch I wonder if we should match the check that follows
> for pcms->cxl_devices_state.is_enabled rather than checking cxl_resv_end
> (which is only set to non 0 if that is_enabled is set).
>
> Probably better to use a consistent condition for checking if CXL is
> there or not.
>
> We also ideally need a suitable fixes tag. I couldn't immediately find one
> so maybe it goes a long way back.
>
Would the following be the correct Fixes tag for this patch?
Fixes: 03b39fcf64bc ("hw/cxl: Make the CXL fixed memory window setup a machine
parameter")
Although earlier versions already had overlapping memory regions, they didn't
affect device 64-bit BAR access.
> > ---
> > hw/i386/pc.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > index 2f58e73d3347..180bc615f3f0 100644
> > --- a/hw/i386/pc.c
> > +++ b/hw/i386/pc.c
> > @@ -975,7 +975,7 @@ void pc_memory_init(PCMachineState *pcms,
> >
> > rom_set_fw(fw_cfg);
> >
> > - if (machine->device_memory) {
> > + if (machine->device_memory || cxl_resv_end) {
> > uint64_t *val = g_malloc(sizeof(*val));
> > uint64_t res_mem_end;
> >
