Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

[Catalin Marinas <catalin.marinas@arm.com>]

+ 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