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Date:   Fri, 8 Sep 2023 17:40:55 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     ankita@nvidia.com
Cc:     jgg@nvidia.com, maz@kernel.org, oliver.upton@linux.dev,
        will@kernel.org, aniketa@nvidia.com, cjia@nvidia.com,
        kwankhede@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com,
        acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com,
        danw@nvidia.com, linux-arm-kernel@lists.infradead.org,
        kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org,
        Lorenzo Pieralisi <lpieralisi@kernel.org>
Subject: Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and
 NORMAL_NC for IO memory
Message-ID: <ZPtOl7EqLzMusuIX@arm.com>
References: <20230907181459.18145-1-ankita@nvidia.com>
 <20230907181459.18145-3-ankita@nvidia.com>
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+ Lorenzo

On Thu, Sep 07, 2023 at 11:14:59AM -0700, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
> 
> Linux allows device drivers to map IO memory on a per-page basis using
> "write combining" or WC. This is often done using
> pgprot_writecombing(). The driver knows which pages can support WC
> access and the proper programming model to generate this IO. Generally
> the use case is to boost performance by using write combining to
> generate larger PCIe MemWr TLPs.
> 
> Allow VMs to select DEVICE_* or NORMAL_NC on a page by page basis for
> all IO memory. This puts the VM in charge of the memory attributes,
> and removes the KVM override to DEVICE_nGnRE.
> 
> Ultimately this makes pgprot_writecombing() work correctly in VMs and
> allows drivers like mlx5 to fully operate their HW.
> 
> After some discussions with ARM and CPU architects we reached the
> conclusion there was no need for KVM to prevent the VM from selecting
> between DEVICE_* and NORMAL_NC for IO memory in VMs. There was a fear
> that NORMAL_NC could result in uncontained failures, but upon deeper
> analysis it turns out there are already possible cases for uncontained
> failures with DEVICE types too. Ultimately the platform must be
> implemented in a way that ensures that all DEVICE_* and NORMAL_NC
> accesses have no uncontained failures.
> 
> Fortunately real platforms do tend to implement this.
> 
> This patch makes the VM's memory attributes behave as follows:
> 
> ?S1?????????? |?? S2????????? |? Result
> ?NORMAL-WB????|? NORMAL-NC????|? NORMAL-NC
> ?NORMAL-WT????|? NORMAL-NC????|? NORMAL-NC
> ?NORMAL-NC????|? NORMAL-NC????|? NORMAL-NC
> ?DEVICE<attr>?|? NORMAL-NC????|? DEVICE<attr>
> 
> See section D8.5.5 of DDI0487_I_a_a-profile_architecture_reference_manual.pdf
> for details.
> 
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>

From the discussions with the hardware people in Arm and Nvidia, we
indeed concluded that relaxing S2 to Normal-NC is not any worse than
Device (and actually Device memory is more prone to generating
uncontained errors, something to do with the Device memory ordering
semantics).

For this change:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

-- 
Catalin