Re: [Qemu-devel] [PATCH v2 1/2] memory: Fix old portio word accesses

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-09-18 18:49, Avi Kivity wrote:
> On 09/18/2011 07:28 PM, Jan Kiszka wrote:
>> >>
>> >>  This is PIO, limited by the x86 address space to 16 bit. Will add a
>> >>  comment.
>> >
>> >  x86 PIO is not limited to 16 bits, just ISA, which memory.c knows
>> >  nothing about.
>>
>> Confused address and data, the former is limited 16, the latter can be
>> 32 as well. But I guess only ISA models made use of the core's split up
>> service, and that's why QEMU limited itself accordingly.
> 
> Let's not bury such details in the core.

It's already in the core (ioport), and would refrain from changing it in
this fix.

> 
>>
>> >
>> >>  >    Why not use access_with_adjusted_size()?
>> >>
>> >>  Because of different accessor prototypes.
>> >>
>> >
>> >  Can be thunked.  There is a different issue, a_w_a_s() can use small
>> >  accesses to emulate large ones, but not vice versa.  It needs fixing
>> >  anyway.
>> >
>>
>> IIRC, that's a feature: Devices not implementing small accesses tend to
>> refuse them in reality.
> 
> I don't think this holds for pci; there the bus always generates 32-bit
> writes with separate byte enables for each lane.  The device need not
> even be aware of a sub-word access, for reads.

The problem is that once we "enhance" the core with such a support to
potentially help one use case, we need to validate all users again if
they depend on the old behavior. That's tricky as breakage may only show
up with odd guests that issue invalid but so far harmless requests.

Jan


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