Re: [PATCH] hw/i386/pc: Avoid overlap between CXL window and PCI 64bit BARs in QEMU

[peng guo <engguopeng@buaa.edu.cn>]

On Fri, Jul 25, 2025 at 02:53:37PM +0100, Jonathan Cameron wrote:
> On Fri, 18 Jul 2025 21:35:45 +0800
> peng guo <engguopeng@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 <engguopeng@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;
> >