Re: [Qemu-devel] [PATCH 1/5] ide/atapi: make PIO read requests async

[Peter Lieven]

Am 08.10.2015 um 18:44 schrieb John Snow:
>
> On 10/08/2015 08:06 AM, Peter Lieven wrote:
>> Hi all,
>>
>> short summary from my side. The whole thing seems to get complicated,
>> let me explain why:
>>
>> 1) During review I found that the code in ide_atapi_cmd_reply_end can't
>> work correctly if the
>> byte_count_limit is not a divider or a multiple of cd_sector_size. The
>> reason is that as soon
>> as we load the next sector we start at io_buffer offset 0 overwriting
>> whatever is left in there
>> for transfer. We also reset the io_buffer_index to 0 which means if we
>> continue with the
>> elementary transfer we always transfer a whole sector (of corrupt data)
>> regardless if we
>> are allowed to transfer that much data. Before we consider fixing this I
>> wonder if it
>> is legal at all to have an unaligned byte_count_limit. It obviously has
>> never caused trouble in
>> practice so maybe its not happening in real life.
>>
> I had overlooked that part. Good catch. I do suspect that in practice
> nobody will be asking for bizarre values.
>
> There's no rule against an unaligned byte_count_limit as far as I have
> read, but suspect nobody would have a reason to use it in practice.
>
>> 2) I found that whatever cool optimization I put in to buffer multiple
>> sectors at once I end
>> up with code that breaks migration because older versions would either
>> not fill the io_buffer
>> as expected or we introduce variables that older versions do not
>> understand. This will
>> lead to problems if we migrate in the middle of a transfer.
>>
> Ech. This sounds like a bit of a problem. I'll need to think about this
> one...
>
>> 3) My current plan to get this patch to a useful state would be to use
>> my initial patch and just
>> change the code to use a sync request if we need to buffer additional
>> sectors in an elementary
>> transfer. I found that in real world operating systems the
>> byte_count_limit seems to be equal to
>> the cd_sector_size. After all its just a PIO transfer an operating
>> system will likely switch to DMA
>> as soon as the kernel ist loaded.
>>
>> Thanks,
>> Peter
>>
> It sounds like that might be "good enough" for now, and won't make
> behavior *worse* than it currently is. You can adjust the test I had
> checked in to not use a "tricky" value and we can amend support for this
> later if desired.

Have you had a chance to look at the series with the "good enough" fix?

Thanks,
Peter