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

[John Snow]

On 10/14/2015 02:19 PM, Peter Lieven wrote:
> 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
> 

Will do so Friday, thanks!

--js