linux-kernel - Re: [PATCH RFC 0/7] Adding core support for wire-MSI bridges

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Date:	Thu, 15 Oct 2015 21:16:15 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	Jason Cooper <jason@...edaemon.net>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ma Jun <majun258@...wei.com>
Subject: Re: [PATCH RFC 0/7] Adding core support for wire-MSI bridges

On Thursday 15 October 2015 17:01:02 Marc Zyngier wrote:
> 
> "Preconfigured" is the key word. While you can do something like that if
> your hardware treats MSIs just as if they were wired interrupts
> (something like GICv2m), it becomes far more hairy if the target of MSIs
> is something like a GICv3 ITS (which is the case for HiSilicon mbigen).
> 
> The main reason is that the ITS relies on "translation tables" kept in
> memory, which the OS has to configure, and handing over pre-configured
> tables is not something I'm looking forward to doing. From a CPU point
> of view, this is akin entering the kernel with the MMU already on and no
> idmap...
> 
> The approach taken here is to make the MSI-ness explicit at the irqchip
> level, and keep the interrupting device oblivious of that feature. Also,
> this relies on the fact that we can have one MSI per wire, meaning that
> we don't have to multiplex anything (no nested irqchip), and that we can
> rely on hierarchical domains, which simplifies the code (at least for
> the irqchip).
> 

Thanks, that already makes things much clearer. Just one more question:
why can't those translation tables be configured statically by the
irqchip driver? Is this all about being able to cut a few cycles
in case of virtualization? I would assume that once you have gone through
the overhead of having both an MSI and a normal interrupt line (with
the need for serialization vs DMA), you can just as well trap to user
space to deliver an IRQ to a guest.

	Arnd
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