linux-kernel - Re: [PATCH v7 09/11] KVM: arm64: guest debug, HW assisted debug support

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Date:	Thu, 2 Jul 2015 09:48:47 +0100
From:	Will Deacon <will.deacon@....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>,
	"jan.kiszka@...mens.com" <jan.kiszka@...mens.com>,
	"dahi@...ux.vnet.ibm.com" <dahi@...ux.vnet.ibm.com>,
	"r65777@...escale.com" <r65777@...escale.com>,
	"bp@...e.de" <bp@...e.de>, Gleb Natapov <gleb@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:ABI/API" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v7 09/11] KVM: arm64: guest debug, HW assisted debug
 support

Hi Alex,

On Wed, Jul 01, 2015 at 07:29:01PM +0100, Alex Bennée wrote:
> This adds support for userspace to control the HW debug registers for
> guest debug. In the debug ioctl we copy an IMPDEF registers into a new
> register set called host_debug_state.
> 
> We use the recently introduced vcpu parameter debug_ptr to select which
> register set is copied into the real registers when world switch occurs.
> 
> I've made some helper functions from hw_breakpoint.c more widely
> available 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.
> 
> Signed-off-by: Alex Bennée <alex.bennee@...aro.org>
> Reviewed-by: Christoffer Dall <christoffer.dall@...aro.org>

[...]

> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> index e7d934d..ac07f2a 100644
> --- a/arch/arm64/kernel/hw_breakpoint.c
> +++ b/arch/arm64/kernel/hw_breakpoint.c
> @@ -50,13 +50,13 @@ static int core_num_brps;
>  static int core_num_wrps;
> 
>  /* Determine number of BRP registers available. */
> -static int get_num_brps(void)
> +int get_num_brps(void)
>  {
>         return ((read_cpuid(ID_AA64DFR0_EL1) >> 12) & 0xf) + 1;
>  }
> 
>  /* Determine number of WRP registers available. */
> -static int get_num_wrps(void)
> +int get_num_wrps(void)
>  {
>         return ((read_cpuid(ID_AA64DFR0_EL1) >> 20) & 0xf) + 1;
>  }

Sorry, just noticed this, but we already have a public interface for
figuring these numbers out as required by perf. Can't you use
hw_breakpoint_slots(...) instead of exposing these internal helpers?

Will
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