linux-kernel - Re: [PATCH v2 00/21] arm64: Virtualization Host Extension support

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Message-ID: <1602215.WjO3PgfQ53@wuerfel>
Date:	Mon, 25 Jan 2016 17:26:17 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH v2 00/21] arm64: Virtualization Host Extension support

On Monday 25 January 2016 16:23:37 Marc Zyngier wrote:
> On 25/01/16 16:15, Arnd Bergmann wrote:
> > On Monday 25 January 2016 15:53:34 Marc Zyngier wrote:
> >> host and guest, reducing the overhead of virtualization.
> >>
> >> In order to have the same kernel binary running on all versions of the
> >> architecture, this series makes heavy use of runtime code patching.
> >>
> >> The first 20 patches massage the KVM code to deal with VHE and enable
> >> Linux to run at EL2. The last patch catches an ugly case when VHE
> >> capable CPUs are paired with some of their less capable siblings. This
> >> should never happen, but hey...
> >>
> >> I have deliberately left out some of the more "advanced"
> >> optimizations, as they are likely to distract the reviewer from the
> >> core infrastructure, which is what I care about at the moment.
> > 
> > One question: as you mention that you use a lot of runtime code patching
> > to make this work transparently, how does this compare to runtime patching
> > the existing kernel to run in EL2 mode without VHE? Is that even possible?
> 
> I haven't explored that particular avenue - by the look of it, this
> would require a lot more work, as v8.0 EL2 lacks a number of features
> that Linux currently requires (like having two TTBRs, for example).

Ok, I see.

> > My interpretation so far as always been "that's too complicated to
> > do because it would require a lot of runtime patching", but now we seem
> > to get that anyway because we want to run a hypervisor-enabled kernel in
> > either EL1 or EL2 depending on the presence of another feature.
> 
> The kernel itself is mostly untouched (what runs at EL1 also runs at EL2
> without any patching, because the new EL2 is now a superset of EL1). It
> is the hypervisor code that gets a beating with the code-patching stick.

Thanks for the explanation, makes sense.

	Arnd