linux-kernel - Re: [PATCH v2] arm64:kgdb: Fix kernel single-stepping

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Message-ID: <20200220145048.GH3704@minyard.net>
Date:   Thu, 20 Feb 2020 08:50:48 -0600
From:   Corey Minyard <minyard@....org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Corey Minyard <cminyard@...sta.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64:kgdb: Fix kernel single-stepping

On Thu, Feb 20, 2020 at 02:21:36PM +0000, Marc Zyngier wrote:
> On 2020-02-19 15:24, minyard@....org wrote:
> > From: Corey Minyard <cminyard@...sta.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...