Re: [Qemu-devel] [PATCH 1/8] rc4030: create custom DMA address space

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/03/2015 20:10, Hervé Poussineau wrote:
> Le 25/03/2015 15:45, Paolo Bonzini a écrit :
>>
>>
>> On 05/03/2015 23:13, Hervé Poussineau wrote:
>>> Add a new memory region in system address space where DMA address space
>>> definition (the 'translation table') belongs, so we can update on the
>>> fly
>>> the DMA address space.
>>>
>>> Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
>>
>> Would it make sense to just use an IOMMU region for the DMA address
>> space?
> 
> I don't really know. Currently, the first user is the dp8393x network
> card (a sysbus device), which is connected through the DMA address space.
> It means that *every* read or write access needs to be translated, even
> those accessing 1 or 2 bytes. dp8393x has no direct connection to the
> system memory.
> On some other machines, like the m68k Quadra 800, the dp8393x is only
> connected to the system memory.
> 
> A second user is the ESP SCSI card in DMA mode, using the DMA address
> space with the help of the DMA controller.
> 
> As DMA address space mapping doesn't change much (except initialization
> phases), and that it is used for small (1 or 2 bytes) and big transfers
> (network packets and disk accesses), I thought that an address space was
> better.

Both are okay.  The IOMMU makes address space changes faster; your
scheme is basically a form of caching, it trades update performance for
improved translation performance.

The trick you're using with shadowing the RAM is nifty, but it leaks a
memory region.  I'll reply to the main patch.

Paolo