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Date: Fri, 23 Feb 2024 17:02:08 +0000
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From: Marc Zyngier <maz@kernel.org>
To: <ankita@nvidia.com>
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Subject: Re: [PATCH v8 0/4] kvm: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory
In-Reply-To: <20240220072926.6466-1-ankita@nvidia.com>
References: <20240220072926.6466-1-ankita@nvidia.com>
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On Tue, 20 Feb 2024 07:29:22 +0000,
<ankita@nvidia.com> wrote:
> 
> From: Ankit Agrawal <ankita@nvidia.com>
> 
> Currently, KVM for ARM64 maps at stage 2 memory that is considered device
> with DEVICE_nGnRE memory attributes; this setting overrides (per
> ARM architecture [1]) any device MMIO mapping present at stage 1,
> resulting in a set-up whereby a guest operating system cannot
> determine device MMIO mapping memory attributes on its own but
> it is always overridden by the KVM stage 2 default.
> 
> This set-up does not allow guest operating systems to select device
> memory attributes independently from KVM stage-2 mappings
> (refer to [1], "Combining stage 1 and stage 2 memory type attributes"),
> which turns out to be an issue in that guest operating systems
> (e.g. Linux) may request to map devices MMIO regions with memory
> attributes that guarantee better performance (e.g. gathering
> attribute - that for some devices can generate larger PCIe memory
> writes TLPs) and specific operations (e.g. unaligned transactions)
> such as the NormalNC memory type.
> 
> The default device stage 2 mapping was chosen in KVM for ARM64 since
> it was considered safer (i.e. it would not allow guests to trigger
> uncontained failures ultimately crashing the machine) but this
> turned out to be asynchronous (SError) defeating the purpose.
> 
> For these reasons, relax the KVM stage 2 device memory attributes
> from DEVICE_nGnRE to Normal-NC.
> 
> Generalizing to other devices may be problematic, however. E.g.
> GICv2 VCPU interface, which is effectively a shared peripheral, can
> allow a guest to affect another guest's interrupt distribution. Hence
> limit the change to VFIO PCI as caution. This is achieved by
> making the VFIO PCI core module set a flag that is tested by KVM
> to activate the code. This could be extended to other devices in
> the future once that is deemed safe.
> 
> [1] section D8.5 - DDI0487J_a_a-profile_architecture_reference_manual.pdf
> 
> Applied over v6.8-rc5.

For the series,

Reviewed-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.