linux-kernel - Re: [PATCH v4 10/12] KVM: arm64: guest debug, HW assisted debug support

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Message-ID: <20150515152346.GB23652@leverpostej>
Date:	Fri, 15 May 2015 16:23:46 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Alex Bennée <alex.bennee@...aro.org>
Cc:	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"christoffer.dall@...aro.org" <christoffer.dall@...aro.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"peter.maydell@...aro.org" <peter.maydell@...aro.org>,
	"agraf@...e.de" <agraf@...e.de>,
	"drjones@...hat.com" <drjones@...hat.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"zhichao.huang@...aro.org" <zhichao.huang@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Russell King <linux@....linux.org.uk>,
	Jonathan Corbet <corbet@....net>,
	Gleb Natapov <gleb@...nel.org>,
	"jan.kiszka@...mens.com" <jan.kiszka@...mens.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:ABI/API" <linux-api@...r.kernel.org>,
	"dahi@...ux.vnet.ibm.com" <dahi@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"r65777@...escale.com" <r65777@...escale.com>,
	"bp@...e.de" <bp@...e.de>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 10/12] KVM: arm64: guest debug, HW assisted debug
 support

Hi Alex,

On Fri, May 15, 2015 at 03:27:13PM +0100, Alex Bennée wrote:
> This adds support for userspace to control the HW debug registers for
> guest debug. In the debug ioctl we copy the IMPDEF defined number of
> registers into a new register set called host_debug_state. There is now
> a new vcpu parameter called debug_ptr which selects which register set
> is to copied into the real registers when world switch occurs.
> 
> I've moved some helper functions into the hw_breakpoint.h header for
> re-use.
> 
> As with single step we need to tweak the guest registers to enable the
> exceptions so we need to save and restore those bits.
> 
> Two new capabilities have been added to the KVM_EXTENSION ioctl to allow
> userspace to query the number of hardware break and watch points
> available on the host hardware.

There's the unfortunate possibility that these could vary across cores
in a big.LITTLE system (though we haven't seen that thus far). The
kernel sanity checks should currently explode if such a case is
encountered, but I don't know what we'd do were that to happen.

This gets more fun when you consider the context-aware breakpoints are
the highest numbered. So the set of (context-aware) breakpoints might
not intersect across all CPUs.

I'm not sure what the best thing to do is w.r.t. exposing that to
userspace.

Thanks,
Mark.
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