Re: [PATCH v6 09/16] iommu/riscv: Reserve an MSI IOVA window for iommufd

From: Nicolin Chen <nicolinc@nvidia.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Andrew Jones <andrew.jones@oss.qualcomm.com>,
	<linux-riscv@lists.infradead.org>, <iommu@lists.linux.dev>,
	<linux-kernel@vger.kernel.org>, <tomasz.jeznach@linux.dev>,
	<tjeznach@rivosinc.com>, <jgg@ziepe.ca>, <joro@8bytes.org>,
	<will@kernel.org>, <robin.murphy@arm.com>, <pjw@kernel.org>,
	<palmer@dabbelt.com>, <anup@brainfault.org>, <tglx@kernel.org>,
	<kevin.tian@intel.com>, <fangyu.yu@linux.alibaba.com>
Subject: Re: [PATCH v6 09/16] iommu/riscv: Reserve an MSI IOVA window for iommufd
Date: Thu, 15 Oct 2026 12:51:33 -0700	[thread overview]
Message-ID: <atEuxafCLCdZrdR7@nvidia.com> (raw)
In-Reply-To: <179209292716.4133959.2758345963634439485.b4-review@b4>

On Thu, Oct 15, 2026 at 04:35:27PM -0300, Jason Gunthorpe wrote:
> > Reserve one page per possible CPU, sufficient for each supervisor IMSIC
> > page. Use a 16 MiB base instead of the 128 MiB convention used by ARM
> > SMMU. kvmtool places its guest IMSIC at 128 MiB, so with IRQ bypass the
> > domain-wide guest MSI address match can also capture host-delivered
> > MSI writes to the SW MSI window and misdeliver them to the guest.
> > 
> > 16 MiB falls in a hole in the current kvmtool and QEMU virt memory maps,
> > away from their guest IMSIC windows and RAM. Keep the window below
> > 4 GiB so devices with 32-bit MSI address registers can use it.
> 
> And this is a good reason you should not follow ARM down this mad
> road. Nicolin had some patches to allow userspace to select the SW_MSI
> range instead of using a kernel hard coded value, which becomes a
> sneaky ABI. It would be much better if riscv starts off on the right
> foot instead of continuing this mistake.

It was paused due to lack of an immediate interest:
https://lore.kernel.org/linux-iommu/cover.1760487869.git.nicolinc@nvidia.com/

If riscv wants it, maybe we can give a push.

Nicolin