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E.g: > > > > qemu stdvga (the default display device) is affected too. > > > > So far, I was only able to verify that the issue has nothing to do with > OVMF and multi-vcpu, it reproduces very well with > > $ qemu-kvm -machine q35,accel=kvm,kernel-irqchip=split -name guest=c10s > -cpu host -smp 1 -m 16384 -drive file=/var/lib/libvirt/images/c10s-bios.qcow2,if=none,id=drive-ide0-0-0 > -device ide-hd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1 > -vnc :0 -device VGA -monitor stdio --no-reboot > > Comparing traces of working and broken cases, I couldn't find anything > suspicious but I may had missed something of course. For now, it seems > like a userspace misbehavior resulting in a segfault. Could you please share steps launch the broken guest desktop? (better also with guest kernel version, name of desktop processes, name of X server) Currently, I couldn't reproduce the error with "-device bochs-display" or "-device VGA" locally on a "Coffee Lake-S" test machine. Qemu cmd as below: qemu-system-x86_64 -m 4096 -smp 1 -M q35 -name guest-01 -hda ubuntu22-1.qcow2 -bios /usr/bin/bios.bin -enable-kvm -k en-us -serial stdio -device bochs-display -machine kernel_irqchip=on -cpu host -usb -usbdevice tablet The guest can see a VGA device 00:02.0 Display controller: Device 1234:1111 (rev 02) with driver # readlink /sys/bus/pci/devices/0000\:00\:02.0/driver ../../../bus/pci/drivers/bochs-drm I have tried hardcoding several fields as below: (1) hardcoded the fb_map to wc in the guest driver --- a/drivers/gpu/drm/tiny/bochs.c +++ b/drivers/gpu/drm/tiny/bochs.c @@ -261,7 +261,9 @@ static int bochs_hw_init(struct drm_device *dev) if (pci_request_region(pdev, 0, "bochs-drm") != 0) DRM_WARN("Cannot request framebuffer, boot fb still active?\n"); - bochs->fb_map = ioremap(addr, size); + bochs->fb_map = ioremap_wc(addr, size); + printk("bochs wc fb_map=%lx, addr=%lx, size=%lx\n", (unsigned long)bochs->fb_map, (unsigned long)addr, (unsigned long)size); if (bochs->fb_map == NULL) { DRM_ERROR("Cannot map framebuffer\n"); return -ENOMEM; With dmesg as below: [ 7.565840] ioremap wc phys_addr fd000000 size 1000000 to wc [ 7.565856] bochs wc fb_map=ffffc90004000000, addr=fd000000, size=1000000 [ 7.565859] [drm] Found bochs VGA, ID 0xb0c5. [ 7.565861] [drm] Framebuffer size 16384 kB @ 0xfd000000, mmio @ 0xfebd9000. [ 7.591995] [drm] Found EDID data blob. [ 7.603956] [drm] Initialized bochs-drm 1.0.0 20130925 for 0000:00:02.0 on minor 1 [ 7.614263] bochs-drm 0000:00:02.0: [drm] fb1: bochs-drmdrmfb frame buffer device (2) hardcoded the memory type to WC in KVM intel driver. + if (gfn >= 0xfd000 && gfn < 0xfe000) + return (MTRR_TYPE_WRCOMB << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT; (3) hardcoded mmap flags to WC for some bo objects for Xorg.