Re: [Qemu-devel] [PATCH 13/19] libqos/ahci: add ahci command size setters

[John Snow]

On 02/02/2015 05:35 AM, Paolo Bonzini wrote:
> On 30/01/2015 19:42, John Snow wrote:
> > +void ahci_command_set_sizes(AHCICommand *cmd, uint64_t xbytes,
> > +                            unsigned prd_size)
> > +{
> > +    /* Each PRD can describe up to 4MiB, and must not be odd. */
> > +    g_assert_cmphex(prd_size, <=, 4096 * 1024);
> > +    g_assert_cmphex(prd_size & 0x01, ==, 0x00);
> > +    cmd->prd_size = prd_size;
> > +    cmd->xbytes = xbytes;
> > +    cmd->fis.count = cpu_to_le16(cmd->xbytes / AHCI_SECTOR_SIZE);
>
> Why do you need cpu_to_le16 here, instead of having it in the function
> that writes the command to guest memory?
>
> Paolo
>
> > +    cmd->header.prdtl = size_to_prdtl(cmd->xbytes, cmd->prd_size);
> > +}
> > +

In this case, only the command header had a utility written for it to 
flip the bits for me. This is part of the FIS, instead, which has no 
explicit flip-on-write mechanism inside of commit.

So, it's correct, but not terribly consistent.

I can write a fis write helper to make this more internally consistent 
about when we handle it for the user and when we don't.