Re: [Qemu-devel] [PATCH 09/16] Enable message delivery via IRQs

[Blue Swirl]

On Sun, Jun 13, 2010 at 3:49 PM, Paul Brook <address@hidden> wrote:
>> I think we could solve all problems (well, maybe not world peace, yet)
>> by switching to message based system for all of DMA and IRQs.
>>
>> Each device would have a message input port and way to output messages.
>>
>> Examples:
>>
>> Zero copy memory access from device D1 to D2 to host memory (D3) with
>> access broken to page length units and errors occurring on the last
>> byte:
>> D1 send_msg(ID, MSG_MEM_WRITE, DMA address, length) -> D2
>>...
>> IRQ delivery chain D1->D2->D3 with coalescing, messages, delivery
>> reporting and EOI:
>> D1 send_msg(ID, MSG_IRQ_RAISE, payload) -> D2
>
> This feels like a terrible idea to me. It introduces an unnecessary RPC
> indirection layer without actually solving any of the problems. It just makes
> it harder (if not impossible) for the compiler to verify any of the interfaces
> between objects.

For the memory access case, in practice the interface could be
sysbus_memory_rw(DeviceState *parent, target_phys_addr_t addr,
target_phys_addr_t size)
in place of send_msg() and
sysbus_memory_rw_cb(DeviceState *dev, void *ptr, size_t size, int status)
in place of send_replymsg() so we'd have compiler type checks.