On Thu, Feb 27, 2020 at 01:04:54PM -0800, Alistair Francis wrote:
The QEMU model for the Bochs display has no VGA memory section at offset 0x400 [1]. By writing to this register Linux can create a write to unassigned memory which depending on machine and architecture can result in a store fault.
I don't see any reference to this address at OSDev [2] or in the Bochs source code.
Removing this write still allows graphics to work inside QEMU with the bochs-display.
It's not that simple. The driver also handles the qemu stdvga (-device VGA, -device secondary-vga) which *does* need the vga port write. There is no way for the guest to figure whenever the device is secondary-vga or bochs-display.
So how about fixing things on the host side? Does qemu patch below help?
Maybe another possible approach is to enable/disable vga access per arch. On x86 this doesn't cause any problems. I guess you are on risc-v?
cheers, Gerd
diff --git a/hw/display/bochs-display.c b/hw/display/bochs-display.c index 62085f9fc063..e93e838243b8 100644 --- a/hw/display/bochs-display.c +++ b/hw/display/bochs-display.c @@ -151,6 +151,26 @@ static const MemoryRegionOps bochs_display_qext_ops = { .endianness = DEVICE_LITTLE_ENDIAN, };
+static uint64_t dummy_read(void *ptr, hwaddr addr, unsigned size) +{ + return -1; +} + +static void dummy_write(void *ptr, hwaddr addr, + uint64_t val, unsigned size) +{ +} + +static const MemoryRegionOps dummy_ops = { + .read = dummy_read, + .write = dummy_write, + .valid.min_access_size = 1, + .valid.max_access_size = 4, + .impl.min_access_size = 1, + .impl.max_access_size = 1, + .endianness = DEVICE_LITTLE_ENDIAN, +}; + static int bochs_display_get_mode(BochsDisplayState *s, BochsDisplayMode *mode) { @@ -284,8 +304,8 @@ static void bochs_display_realize(PCIDevice *dev, Error **errp) memory_region_init_io(&s->qext, obj, &bochs_display_qext_ops, s, "qemu extended regs", PCI_VGA_QEXT_SIZE);
- memory_region_init(&s->mmio, obj, "bochs-display-mmio", - PCI_VGA_MMIO_SIZE); + memory_region_init_io(&s->mmio, obj, &dummy_ops, NULL, + "bochs-display-mmio", PCI_VGA_MMIO_SIZE); memory_region_add_subregion(&s->mmio, PCI_VGA_BOCHS_OFFSET, &s->vbe); memory_region_add_subregion(&s->mmio, PCI_VGA_QEXT_OFFSET, &s->qext);