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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH 09/16] Enable message delivery via IRQs
Date: Sun, 13 Jun 2010 19:39:08 +0100
References: <cover.1275811861.git.jan.kiszka@web.de>
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To: Blue Swirl <blauwirbel@gmail.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>, Jan Kiszka <jan.kiszka@web.de>, qemu-devel@nongnu.org, Gleb Natapov <gleb@redhat.com>, Juan Quintela <quintela@redhat.com>

> On Sun, Jun 13, 2010 at 3:49 PM, Paul Brook <paul@codesourcery.com> 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)

Why "parent"?

> 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.

I don't see any point point trying to squeeze this through a common message 
passing API. We *could* do that if we really wanted, but It's a lot of hassle. 
If devices are going to end up using wrappers that look a lot like a straight 
API then what's the point?

Paul