Re: [Qemu-devel] [PATCH 14/16] ahci: Do not map cmd_fis to generate response

[John Snow]

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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.

The field is supposed to contain the contents of the sector count
register at the conclusion of the command. The spec says that for data
transfer commands, this value is useless/uninteresting (it can be
zero-filled, or filled with garbage, etc)

However, for some special purpose commands, the sector count register
is required to hold a value that has some special meaning. I don't
think we currently support any of those, but for the future -- this
part of the FIS generation code will work correctly regardless.

So this patch is mainly about getting rid of the cmd_fis mapping and
putting something nominally correct in its place.

- --js
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