Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 20 Feb 2020 08:50:48 -0600
From:   Corey Minyard <minyard@acm.org>
To:     Marc Zyngier <maz@kernel.org>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        linux-arm-kernel@lists.infradead.org,
        Corey Minyard <cminyard@mvista.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] arm64:kgdb: Fix kernel single-stepping
Message-ID: <20200220145048.GH3704@minyard.net>
Reply-To: minyard@acm.org
References: <20200219152403.3495-1-minyard@acm.org>
 <1416dca51b52dff349923184f41d48e8@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1416dca51b52dff349923184f41d48e8@kernel.org>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Feb 20, 2020 at 02:21:36PM +0000, Marc Zyngier wrote:
> On 2020-02-19 15:24, minyard@acm.org wrote:
> > From: Corey Minyard <cminyard@mvista.com>
> 
> [...]
> 
> > After studying the EL0 handling for this, I realized an issue with using
> > MDSCR to check if single step is enabled: it can be expensive on a VM.
> > So check the task flag first to see if single step is enabled.  Then
> > check MDSCR if the task flag is set.
> 
> Very tangential remark: I'd really like people *not* to try and optimize
> Linux based on the behaviour of a hypervisor. In general, reading a
> system register is fast, and the fact that it traps on a given hypervisor
> at some point may not be true in the future, nor be a valid assumption
> across hypervisors.

Normally I would agree, but I based this upon git commit
https://github.com/torvalds/linux/commit/2a2830703a2371b47f7b50b1d35cb15dc0e2b717
which seemed to say that it was a significant enough factor to do in the
EL0 case.

-corey

> 
>         M.
> -- 
> Jazz is not dead. It just smells funny...