Re: [PATCH 2/2] virtio_scsi: Implement fc_host

[Michael S. Tsirkin]

On Tue, Jan 17, 2017 at 10:05:00PM +0800, Fam Zheng wrote:
> On Tue, 01/17 14:17, Paolo Bonzini wrote:
> > 
> > 
> > On 16/01/2017 18:26, Fam Zheng wrote:
> > >> Is the endianness correct for big-endian host here?
> > >
> > > I think so. The fc_host sysfs uses u64 to represent port_name and node_name,
> > > this patch does the same, so using virtio_* helpers for these fields should
> > > handle the endianness correctly.
> > 
> > I was suspicious about it because they are defined as "u8 x[8]" in the
> > virtio_scsi_config struct.  So you would need to read with
> > virtio_cread_bytes and pass the result to wwn_to_u64.
> > 
> > For example, if you have 0x500123456789abcd this would be
> > 
> > 	0x50 0x01 0x23 0x45 0x67 0x89 0xab 0cd
> > 
> > in virtio_scsi_config, and then virtio_cread64 would read it as a
> > little-endian u64, 0xcdab896745230150.  Maybe your QEMU patch is also
> > writing things as little-endian 64-bit integers, rather than 8-element
> > arrays of bytes?
> 
> Yes, they all used 64-bit integers in a "less surprising" endian. I think there
> is an endianness conecpt to WWN, as in 0x500123456789abcd; and there is an
> native endianness to virtio, which is little-endian. If we use a "u8 x[8]" type
> in the spec and want the WWN's MSB, namely the 0x50 stuff, to be the first byte,
> is it worth to explicitly document that to avoid confusion?
> 
> Fam

Can't hurt, for sure.