Re: [Qemu-devel] [PULL 3/5] exec: Support 64-bit operations in address_space_rw

[Paolo Bonzini]

Il 17/07/2013 15:23, Richard Henderson ha scritto:
> On 07/17/2013 04:09 AM, Paolo Bonzini wrote:
>>>
>>> Fails for me:
>>>
>>> qemu-system-x86_64: /work/armbru/qemu/exec.c:1927: memory_access_size: 
>>> Assertion `l >= access_size_min' failed.
>>
>> This:
>>
>>     unsigned access_size_min = mr->ops->impl.min_access_size;
>>     unsigned access_size_max = mr->ops->impl.max_access_size;
>>
>> must be respectively:
>>
>>     unsigned access_size_min = 1;
>>     unsigned access_size_max = mr->ops->valid.max_access_size;
>>
>> access_size_min can be 1 because erroneous accesses must not crash 
>> QEMU, they should trigger exceptions in the guest or just return 
>> garbage (depending on the CPU).  I'm not sure I understand the comment, 
>> placing a 4-byte field at the last byte of a region makes no sense 
>> (unless impl.unaligned is true).
>>
>> access_size_max can be mr->ops->valid.max_access_size because memory.c 
>> can and will still break accesses bigger than 
>> mr->ops->impl.max_access_size.
>>
>> Markus, can you try the minimal patch above?  Or this one that also
>> does the consequent simplifications.
> 
> NAK.
> 
> If you remove the check here, you're just trading it for one in the device.
> The device told you that it can't support a 1 byte read.  (Either that, or the
> device incorrectly reported what it can actually do.)

There are two parts to this.

First of all, mr->ops->impl.min_access_size is definitely wrong.  The
device told me that the MMIO functions only know about 2-byte accesses,
but that it _can_ support 1-, 2- and 4- byte reads (with coalescing done
by memory.c).  So I could change access_size_min to
mr->ops->valid.min_access_size, which would also fix Markus's problem.

But then, accesses smaller than mr->ops->valid.min_access_size are fine,
they just result in exceptions or garbage reads (depending on the CPU).
 address_space_rw reports these errors just fine,  memory_access_size's
only purpose is to split address_space_rw's MMIO writes in a sensible
manner.  There is no error reporting because it is done in memory.c.

In fact, I'm not even sure if users of memory_access_size (DMA to an
MMIO destination) exist in real hardware.  I'm curious if "BSAVE"ing
16-color EGA graphics works with a modern graphic card and a BIOS that
doesn't use PIO.

Paolo

> The proper fix is to change the interface of memory_access_size such that it
> can report errors.  Indeed, very likely we should change it and its callers to
> also support over-sized reads, like access_with_adjusted_size in memory.c.
> 
> 
> r~
>