On Mon, Jun 29, 2015 at 11:07:18AM -0400, John Snow wrote: > On 06/29/2015 10:51 AM, Stefan Hajnoczi wrote: > > On Fri, Jun 26, 2015 at 01:31:12PM -0400, John Snow wrote: > >> On 06/26/2015 11:59 AM, Stefan Hajnoczi wrote: > >>> On Mon, Jun 22, 2015 at 08:21:13PM -0400, John Snow wrote: > >>>> @@ -744,8 +722,8 @@ static void ahci_write_fis_pio(AHCIDevice > >>>> *ad, uint16_t len) pio_fis[9] = s->hob_lcyl; pio_fis[10] = > >>>> s->hob_hcyl; pio_fis[11] = 0; - pio_fis[12] = cmd_fis[12]; - > >>>> pio_fis[13] = cmd_fis[13]; + pio_fis[12] = s->nsector & 0xFF; > >>>> + pio_fis[13] = (s->nsector >> 8) & 0xFF; > >>> > >>> hw/ide/core.c decreases s->nsector until it reaches 0 and the > >>> request ends. > >>> > >>> Will the values reported back to the guest be correct if we use > >>> s->nsector? > >>> > >> > >> See the commit message for justification of this one. Ultimately, it > >> doesn't matter what gets put in here (for data transfer commands) -- > >> but getting RID of the cmd_fis mapping is a strong positive. > > > > Getting rid of cmd_fis mapping is good. > > > > Putting s->nsector into the undefined fields makes the code confusing. > > > > It is clearer to zero the bytes with a comment saying the value does not > > matter according to the spec. > > > > Well, it's not that it doesn't matter /ever/, it's more that for > standard IO routines it doesn't matter. (See the normative output spec > in ATA8-AC3 -- for most cases it's N/A, but for a handful of cases it > carries a diagnostic signature.) > > What's really the case is that the FIS always dutifully copies out what > the SATA registers are (or should be.) > > There are still a handful of commands that, if we choose to support > them, copying the nsector register would be the "correct thing" to do, > so I decided to copy that field here to serve as documentation and > support future command additions. > > I would argue that if this field ever does the /wrong thing/, it would > be a fix in the S/ATA layer, and not a change to the FIS generator here. > > I am inclined to leave it as-is, since for the current cases, nsector is > going to empty to zero anyway. I believe the behavior presented here is > correct.
I'm trying to understand the guest-visible change in behavior. Guests might take different code paths from before. For example, I think that after this patch the CHECK POWER MODE command works correctly for the first time with AHCI. It sets ->nsector to 0xff. Anyway, I'm happy with assigning s->nsector.
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