Re: [PULL 33/75] hw/i386/pc: Avoid overlap between CXL window and PCI 64bit BARs in QEMU

[Jonathan Cameron via <qemu-devel@nongnu.org>]

On Tue, 28 Oct 2025 22:26:12 +0300
Michael Tokarev <mjt@tls.msk.ru> wrote:

> On 10/6/25 20:08, Michael Tokarev wrote:
> > On 10/5/25 22:17, Michael S. Tsirkin wrote:  
> >> From: peng guo <engguopeng@buaa.edu.cn>
> >>
> >> When using a CXL Type 3 device together with a virtio 9p device in 
> >> QEMU on a
> >> physical server, the 9p device fails to initialize properly. The 
> >> kernel reports
> >> the following error:
> >>
> >>      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 adds the reserved memory end 
> >> calculation
> >> for cxl devices 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
> >>
> >> Fixes: 03b39fcf64bc ("hw/cxl: Make the CXL fixed memory window setup a 
> >> machine parameter")
> >> Signed-off-by: peng guo <engguopeng@buaa.edu.cn>
> >> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> >> Message-ID: <20250805142300.15226-1-engguopeng@buaa.edu.cn>
> >> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> >> ---
> >>   hw/i386/pc.c | 20 +++++++++++---------
> >>   1 file changed, 11 insertions(+), 9 deletions(-)  
> > 
> > Hi!
> > 
> > Is it qemu-stable material (10.0.x & 10.1.x)?  
> 
> A friendly ping for the stable series?

I think it does make sense for stable.

Thanks,

Jonathan

> 
> Thanks,
> 
> /mjt
>