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

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Message-ID: <1416dca51b52dff349923184f41d48e8@kernel.org>
Date:   Thu, 20 Feb 2020 14:21:36 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     minyard@....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 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.

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