linux-kernel - Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report

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Message-ID: <20250123134226.GO5556@nvidia.com>
Date: Thu, 23 Jan 2025 09:42:26 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>, "corbet@....net" <corbet@....net>,
	"will@...nel.org" <will@...nel.org>,
	"joro@...tes.org" <joro@...tes.org>,
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	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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	"Liu, Yi L" <yi.l.liu@...el.com>,
	"patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report_event
 helper

On Wed, Jan 22, 2025 at 11:54:52AM -0800, Nicolin Chen wrote:

> ARM seems to also have an interesting event merging feature:
>  STE.MEV, bit [83]
>  Merge Events arising from terminated transactions from this stream.
>    0b0 Do not merge similar fault records
>    0b1 Permit similar fault records to be merged
>  The SMMU might be able to reduce the usage of the Event queue by
>  coalescing fault records that share the same page granule of address,
>  access type and SubstreamID.
>    Setting MEV == 1 does not guarantee that faults will be coalesced.
>    Setting MEV == 0 causes a physical SMMU to prevent coalescing of
>    fault records, however, a hypervisor might not honour this setting
>    if it deems a guest to be too verbose.
>  Note: Software must expect, and be able to deal with, coalesced fault
>  records even when MEV == 0.
> 
> Yet, the driver doesn't seem to care setting it at this moment.

Yeah, we will eventually need to implement whatever DOS mitigations
are included in the IOMMU architectures..

I think DOS testing the event architecture should be part of the
testing/qualification.

It should be quite easy to make a DOS spammer using VFIO in userspace
in the VM to test it with the mlx5 vfio driver.. (though you need VFIO
to work in a VM which RMR will prevent, but that can be hacked around
I guess)

Jason