[Qemu-devel] PCI 64-bit BAR access with qemu

[Francois WELLENREITER]

Hi there,

    I've read a few days ago that it was possible to emulate PCI device 
with 64-bit BARs and have a real 64-bit memory access.
Thus, I've created a virtual device named toto accessible through a 
64-bit BAR

___

static const MemoryRegionOps bxi_common_mmio_ops = {
    .read = toto_mmio_read,
    .write = toto_mmio_write,
    .endianness = DEVICE_LITTLE_ENDIAN,
    .impl = {
            .min_access_size = 1,
            .max_access_size = 8,
    },
};

___

    memory_region_init_io(&d->mmio, &toto_mmio_ops, d, "toto-mmio",
        0x1000);

    pci_register_bar(&d->dev, BAR_0,
             PCI_BASE_ADDRESS_SPACE_MEMORY | PCI_BASE_ADDRESS_MEM_TYPE_64,
&d->mmio);

____

when I use in my driver pointers to "unsigned char", "unsigned short" or 
"unsigned int",
I can see that toto_mmio_read is called with increasing sizes.
But that does not work at all with unsigned long long.
In such a case, I always obtain two calls to toto_mmio_read with sizes 
limited to 4.
My understanding here is that a direct 64-bit memory access does not 
work (but is composed of 2 32-bit accesses).

Then, I've tried to understand why this happens and realized that 
cpu_physical_memory_rw was always called for this memory region (for in 
the old_mmio manner) :

___

(gdb) bt
#0  toto_mmio_read (opaque=0x2bec2c0, addr=0, size=4) at 
/home/workspace/qemu/hw/toto_hw.c:92
#1  0x00000000005f6d37 in memory_region_read_accessor (opaque=0x2bec738, 
addr=0, value=0x7f9997ffec78, size=
    4, shift=0, mask=4294967295) at /home/workspace/qemu/memory.c:239
#2  0x00000000005f6ee0 in access_with_adjusted_size (addr=0, 
value=0x7f9997ffec78, size=4, access_size_min=1,
    access_size_max=8, access=0x5f6cdf <memory_region_read_accessor>, 
opaque=0x2bec738)
    at /home/workspace/qemu/memory.c:284
#3  0x00000000005f8909 in memory_region_read_thunk_n (_mr=0x2bec738, 
addr=0, size=4)
    at /home/workspace/qemu/memory.c:824
#4  0x00000000005f8af1 in memory_region_read_thunk_l (mr=0x2bec738, addr=0)
    at /home/workspace/qemu/memory.c:867
#5  0x00000000005c8f69 in cpu_physical_memory_rw (addr=3892314112, 
buf=0x7f999c5d0028 "\320\b", len=8,
    is_write=0) at /home/workspace/qemu/exec.c:3965
#6  0x00000000005ecfa1 in kvm_cpu_exec (env=0x238ee10) at 
/home/workspace/qemu/kvm-all.c:985
#7  0x00000000005bf1fb in qemu_kvm_cpu_thread_fn (arg=0x238ee10) at 
/home/workspace/qemu/cpus.c:661
#8  0x00000034dfc077e1 in start_thread () from /lib64/libpthread.so.0
#9  0x00000034df4e68ed in clone () from /lib64/libc.so.6

____

Did I miss anything ? Is it a real defect ?
Are there developments still planned to allow a direct 64-bit access ?

Thanks for any help,

                             François