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

[Marc Zyngier]

On 2020-02-20 14:50, Corey Minyard wrote:
> On Thu, Feb 20, 2020 at 02:21:36PM +0000, Marc Zyngier wrote:
> > On 2020-02-19 15:24, minyard@xxxxxxx wrote:
> > > From: Corey Minyard <cminyard@xxxxxxxxxx>
> >
> > [...]
> >
> > > 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.

And that's a blast from a distant past. Hypervisors have changed 
drastically
over these 6 years, and I'm still sitting on a bunch of patches that 
*could*
change the way MDSCR_EL1 is handled.

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