linux-kernel - Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL

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Message-ID: <ZSliChc3poZNM4f9@lpieralisi>
Date:   Fri, 13 Oct 2023 17:28:10 +0200
From:   Lorenzo Pieralisi <lpieralisi@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will@...nel.org>, Jason Gunthorpe <jgg@...dia.com>,
        ankita@...dia.com, maz@...nel.org, oliver.upton@...ux.dev,
        aniketa@...dia.com, cjia@...dia.com, kwankhede@...dia.com,
        targupta@...dia.com, vsethi@...dia.com, acurrid@...dia.com,
        apopple@...dia.com, jhubbard@...dia.com, danw@...dia.com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and
 NORMAL_NC for IO memory

On Fri, Oct 13, 2023 at 02:08:10PM +0100, Catalin Marinas wrote:

[...]

> Yes, we end up with mismatched aliases but they only matter if the VMM
> also accesses the I/O range via its own mapping. So far I haven't seen
> case that suggests this.
> 
> > > Things can go wrong but that's not because Device does anything better.
> > > Given the RAS implementation, external aborts caused on Device memory
> > > (e.g. wrong size access) is uncontainable. For Normal NC it can be
> > > contained (I can dig out the reasoning behind this if you want, IIUC
> > > something to do with not being able to cancel an already issued Device
> > > access since such accesses don't allow speculation due to side-effects;
> > > for Normal NC, it's just about the software not getting the data).
> > 
> > I really think these details belong in the commit message.
> 
> I guess another task for Lorenzo ;).

I will do, I start wondering though whether this documentation belongs
in this commit log only or at Documentation/arch/arm64 level (or both),
I am pretty sure this thread can turn out quite useful as a reference
(it is for me) if we manage to summarize it that would benefit
everyone.

Lorenzo