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Date:   Thu, 20 Feb 2020 14:21:36 +0000
From:   Marc Zyngier <maz@kernel.org>
To:     minyard@acm.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
In-Reply-To: <20200219152403.3495-1-minyard@acm.org>
References: <20200219152403.3495-1-minyard@acm.org>
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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.

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