Re: [Qemu-devel] [PATCH] vfio: Make BARs native endian

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 06/19/2014 04:57 AM, Alex Williamson wrote:
> On Wed, 2014-06-18 at 21:35 +1000, Alexey Kardashevskiy wrote:
>> Slow BAR access path is used when VFIO fails to mmap() BAR or TCG is used.
> 
> TCG can use the mmap too

Oh, right.


> 
>> Since this is just a transport between the guest and a device, there is
>> no need to do endianness swapping.
>>
>> This changes BARs to use native endianness. Since non-ROM BARs were
>> doing byte swapping, we need to remove it so does the patch. No change
>> in behavior is expected though.
>>
>> ROM BARs were declared little endian but byte swapping was not
>> implemented for them so they never actually worked on big endian systems.
>> This fixes endiannes for ROM BARs: this declares them native endian and
>> fixes access sizes.
> 
> So the only actual behavior change we expect from this is to fix ROM
> access on big endian where we had an unbalanced set of byte swaps.  BAR
> access worked because the byte swapping in the read/write handler
> canceled the byte swapping in QEMU because we declared the BARs as
> little endian.  Otherwise we're just removing canceling byte swaps for
> big endian.  Is that correct?

Yes.

> 
> Also, as the kernel patch appears to not produce any behavioral change,
> only eliminates canceling swaps, there's no dependency between patches,
> right?  Thanks,

That is right.

> 
> Alex
> 
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>>  hw/misc/vfio.c | 41 +++++++++++++++++++++++++++++++----------
>>  1 file changed, 31 insertions(+), 10 deletions(-)
>>
>> diff --git a/hw/misc/vfio.c b/hw/misc/vfio.c
>> index 7437c2e..3eb3c71 100644
>> --- a/hw/misc/vfio.c
>> +++ b/hw/misc/vfio.c
>> @@ -1052,10 +1052,10 @@ static void vfio_bar_write(void *opaque, hwaddr addr,
>>          buf.byte = data;
>>          break;
>>      case 2:
>> -        buf.word = cpu_to_le16(data);
>> +        buf.word = data;
>>          break;
>>      case 4:
>> -        buf.dword = cpu_to_le32(data);
>> +        buf.dword = data;
>>          break;
>>      default:
>>          hw_error("vfio: unsupported write size, %d bytes", size);
>> @@ -1112,10 +1112,10 @@ static uint64_t vfio_bar_read(void *opaque,
>>          data = buf.byte;
>>          break;
>>      case 2:
>> -        data = le16_to_cpu(buf.word);
>> +        data = buf.word;
>>          break;
>>      case 4:
>> -        data = le32_to_cpu(buf.dword);
>> +        data = buf.dword;
>>          break;
>>      default:
>>          hw_error("vfio: unsupported read size, %d bytes", size);
>> @@ -1142,7 +1142,7 @@ static uint64_t vfio_bar_read(void *opaque,
>>  static const MemoryRegionOps vfio_bar_ops = {
>>      .read = vfio_bar_read,
>>      .write = vfio_bar_write,
>> -    .endianness = DEVICE_LITTLE_ENDIAN,
>> +    .endianness = DEVICE_NATIVE_ENDIAN,
>>  };
>>  
>>  static void vfio_pci_load_rom(VFIODevice *vdev)
>> @@ -1204,21 +1204,42 @@ static void vfio_pci_load_rom(VFIODevice *vdev)
>>  static uint64_t vfio_rom_read(void *opaque, hwaddr addr, unsigned size)
>>  {
>>      VFIODevice *vdev = opaque;
>> -    uint64_t val = ((uint64_t)1 << (size * 8)) - 1;
>> +    union {
>> +        uint8_t byte;
>> +        uint16_t word;
>> +        uint32_t dword;
>> +        uint64_t qword;
>> +    } buf;
>> +    uint64_t data = 0;
>>  
>>      /* Load the ROM lazily when the guest tries to read it */
>>      if (unlikely(!vdev->rom && !vdev->rom_read_failed)) {
>>          vfio_pci_load_rom(vdev);
>>      }
>>  
>> -    memcpy(&val, vdev->rom + addr,
>> +    memcpy(&buf, vdev->rom + addr,
>>             (addr < vdev->rom_size) ? MIN(size, vdev->rom_size - addr) : 0);
>>  
>> +    switch (size) {
>> +    case 1:
>> +        data = buf.byte;
>> +        break;
>> +    case 2:
>> +        data = buf.word;
>> +        break;
>> +    case 4:
>> +        data = buf.dword;
>> +        break;
>> +    default:
>> +        hw_error("vfio: unsupported read size, %d bytes", size);
>> +        break;
>> +    }
>> +
>>      DPRINTF("%s(%04x:%02x:%02x.%x, 0x%"HWADDR_PRIx", 0x%x) = 0x%"PRIx64"\n",
>>              __func__, vdev->host.domain, vdev->host.bus, vdev->host.slot,
>> -            vdev->host.function, addr, size, val);
>> +            vdev->host.function, addr, size, data);
>>  
>> -    return val;
>> +    return data;
>>  }
>>  
>>  static void vfio_rom_write(void *opaque, hwaddr addr,
>> @@ -1229,7 +1250,7 @@ static void vfio_rom_write(void *opaque, hwaddr addr,
>>  static const MemoryRegionOps vfio_rom_ops = {
>>      .read = vfio_rom_read,
>>      .write = vfio_rom_write,
>> -    .endianness = DEVICE_LITTLE_ENDIAN,
>> +    .endianness = DEVICE_NATIVE_ENDIAN,
>>  };
>>  
>>  static bool vfio_blacklist_opt_rom(VFIODevice *vdev)
> 
> 
> 


-- 
Alexey