linux-kernel - Re: [PATCH v2] kvm/arm64: fixed passthrough gpu into vm on arm64

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Date:   Mon, 4 Apr 2022 10:24:05 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Marc Zyngier <maz@...nel.org>
Cc:     xieming <xieming@...inos.cn>, sashal@...nel.org,
        catalin.marinas@....com, linux@...linux.org.uk,
        linux-kernel@...r.kernel.org, alex.williamson@...hat.com,
        will@...nel.org, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] kvm/arm64: fixed passthrough gpu into vm on arm64

On Fri, Apr 01, 2022 at 05:48:59PM +0100, Marc Zyngier wrote:

> NAK. For a start, there is no such thing as 'write-combine' in the ARM
> architecture, and I'm not convinced you can equate WC to Normal-NC.
> See the previous discussion at [1].
> 
> [1] https://lore.kernel.org/r/20210429162906.32742-1-sdonthineni@nvidia.com

We've had a lot of discussions with ARM related to how this works with
drivers like mlx5 that use WC.

ARM has now published some guidance on this:

https://community.arm.com/arm-research/m/resources/1012

As an ecosystem we seem to be drifting toward Normal-NC for this
behavior (largely because it is what Linux does). At least that is
what we are testing and qualifing ARM CPUs against mlx5 with.

I'm guessing it will turn into a SBSA like thing where the ARM ARM is
kind of vauge but a SOC has to implement Normal-NC in a certain way to
be functional for the server market.

Jason