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

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

On Fri, Jul 18, 2025 at 09:35:45PM +0800, peng guo 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>

Cc Jonathan.
Any input on this?


> ---
>  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;
>  
> -- 
> 2.43.0