linux-kernel - Re: [RFC PATCH 1/4] ARM: KVM: on unhandled IO mem abort, route the call to the KVM MMIO bus

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Message-ID: <20141113112012.GO19598@cbox>
Date:	Thu, 13 Nov 2014 12:20:12 +0100
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Nikolay Nikolaev <n.nikolaev@...tualopensystems.com>
Cc:	Andre Przywara <andre.przywara@...aro.org>,
	Antonios Motakis <a.motakis@...tualopensystems.com>,
	kvmarm@...ts.cs.columbia.edu,
	VirtualOpenSystems Technical Team <tech@...tualopensystems.com>,
	Alexander Graf <agraf@...e.de>, marc.zyngier@....com,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	"open list:KERNEL VIRTUAL MA..." <kvm@...r.kernel.org>,
	"moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/4] ARM: KVM: on unhandled IO mem abort, route the
 call to the KVM MMIO bus

On Thu, Nov 13, 2014 at 12:45:42PM +0200, Nikolay Nikolaev wrote:

[...]

> >>
> >> Going through the vgic_handle_mmio we see that it will require large
> >> refactoring:
> >>  - there are 15 MMIO ranges for the vgic now - each should be
> >> registered as a separate device
> >>  - the handler of each range should be split into read and write
> >>  - all handlers take 'struct kvm_exit_mmio', and pass it to
> >> 'vgic_reg_access', 'mmio_data_read' and 'mmio_data_read'
> >>
> >> To sum up - if we do this refactoring of vgic's MMIO handling +
> >> kvm_io_bus_ API getting 'vcpu" argument we'll get a 'much' cleaner
> >> vgic code and as a bonus we'll get 'ioeventfd' capabilities.
> >>
> >> We have 3 questions:
> >>  - is the kvm_io_bus_ getting 'vcpu' argument acceptable for the other
> >> architectures too?
> >>  - is this huge vgic MMIO handling redesign acceptable/desired (it
> >> touches a lot of code)?
> >>  - is there a way that ioeventfd is accepted leaving vgic.c in it's
> >> current state?
> >>
> > Not sure how the latter question is relevant to this, but check with
> > Andre who recently looked at this as well and decided that for GICv3 the
> > only sane thing was to remove that comment for the gic.
> @Andre - what's your experience with the GICv3 and MMIO handling,
> anything specific?
> >
> > I don't recall the details of what you were trying to accomplish here
> > (it's been 8 months or so) but the surely the vgic handling code should
> > *somehow* be integrated into the handle_kernel_mmio (like Paolo
> > suggested), unless you come back and tell me that that would involve a
> > complete rewrite of the vgic code.
> I'm experimenting now - it's not exactly rewrite of whole vgic code,
> but it will touch a lot of it  - all MMIO access handlers and the
> supporting functions.
> We're ready to spend the effort. My question is  - is this acceptable?
> 
I certainly appreciate the offer to do this work, but it's hard to say
at this point if it is worth it.

What I was trying to say above is that Andre looked at this, and came to
the conclusion that it is not worth it.

Marc, what are your thoughts?

-Christoffer
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