linux-kernel - Re: [PATCH v2 05/10] KVM: arm: introduce kvm_arch_setup/clear

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Date:	Mon, 13 Apr 2015 16:48:43 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Alex Bennée <alex.bennee@...aro.org>
Cc:	kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
	peter.maydell@...aro.org, agraf@...e.de, drjones@...hat.com,
	pbonzini@...hat.com, zhichao.huang@...aro.org,
	jan.kiszka@...mens.com, dahi@...ux.vnet.ibm.com,
	r65777@...escale.com, bp@...e.de, Gleb Natapov <gleb@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Andre Przywara <andre.przywara@....com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 05/10] KVM: arm: introduce kvm_arch_setup/clear_debug()

On Mon, Apr 13, 2015 at 04:36:23PM +0200, Christoffer Dall wrote:

[...]

> > +
> > +/**
> > + * kvm_arch_setup_debug - set-up debug related stuff
> 
> nit: I think you want "set up" when it's a verb.
> 
> > + *
> > + * @vcpu:	the vcpu pointer
> > + *
> > + * This is called before each entry in to the hypervisor to setup any
> 
> s/in to/into/
> s/setup/set up/
> 
> > + * debug related registers. Currently this just ensures we will trap
> > + * access to:
> 
> guest accesses to:
> 
> > + *  - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> > + *  - Debug ROM Address (MDCR_EL2_TDRA)
> > + *  - Power down debug registers (MDCR_EL2_TDOSA)
> > + *
> > + * Additionally the hypervisor lazily saves/restores the debug
> > + * register state. If it is not currently doing so (arch.debug_flags)
> > + * then we also need to ensure we trap if the guest messes with them
> > + * so we know we need to save them.
> 
> This paragraph is a little hard to make sense of.  If I understand it
> correctly, the point is that when debugging the guest we need to make
> sure guest accesses to the debug registers traps?  If so, I would
> suggest something like:
> 
>   Additionally, KVM only traps guest accesses to the debug registers if
>   the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
>   flag on vcpu->arch.debug_flags).  Since the guest must not interfere
>   with the hardware state when debugging the guest, we must ensure that
>   trapping is enabled whenever we are debugging the guest.
> 

thinking about this, I don't think we're enforcing this yet, but maybe
that will come in the later patches or I misread the original paragraph.

-Christoffer
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