Re: [RFC PATCH v2 08/18] iommu/riscv: Use MSI table to enable IMSIC access

From: Andrew Jones <ajones@ventanamicro.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	iommu@lists.linux.dev,  kvm-riscv@lists.infradead.org,
	kvm@vger.kernel.org, linux-riscv@lists.infradead.org,
	 linux-kernel@vger.kernel.org, zong.li@sifive.com,
	tjeznach@rivosinc.com, joro@8bytes.org,  will@kernel.org,
	robin.murphy@arm.com, anup@brainfault.org, atish.patra@linux.dev,
	 alex.williamson@redhat.com, paul.walmsley@sifive.com,
	palmer@dabbelt.com, alex@ghiti.fr
Subject: Re: [RFC PATCH v2 08/18] iommu/riscv: Use MSI table to enable IMSIC access
Date: Tue, 23 Sep 2025 10:37:30 -0500	[thread overview]
Message-ID: <20250923-54e8e0f39d672845e2979286@orel> (raw)
In-Reply-To: <20250923145251.GP1391379@nvidia.com>

On Tue, Sep 23, 2025 at 11:52:51AM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 23, 2025 at 09:37:31AM -0500, Andrew Jones wrote:
> > undergoes a specified translation into an index of the MSI table. For the
> > non-virt use case we skip the "composes a new address/data pair, which
> > points at the remap table entry" step since we just forward the original
> > with an identity mapping. For the virt case we do write a new addr,data
> > pair (Patch15) since we need to map guest addresses to host addresses (but
> > data is still just forwarded since the RISC-V IOMMU doesn't support data
> > remapping). 
> 
> You should banish thinking of non-virt/virt from your lexicon. Linux
> doesn't work that way, and trying to force it too is a loosing battle.

Well, we need to consider virt when the hardware has virt-specific
features that we want to control. We also need to consider virt when
additional address translations to go from guest space to host space
are needed, as in this case.

> 
> If you have a remap domain then it should always be remapping. There
> is no such idea in Linux as a conditional IRQ domain dependent on
> external factors (like how the IOMMU is configured, if the device is
> "virt" or not, etc).

The remap domain is created when the platform supports MSIs and always
does remapping when the IOMMU supports the MSI table. It could even do
remapping when the IOMMU doesn't support the MSI table since it could
use the DMA table instead, but that's left for a later patch series if
hardware actually shows up like that.

The difference between virt and non-virt is what addresses get remapped
for the remapping. For virt, guest addresses get remapped, for non-virt,
we don't remap guest addresses. And, since we don't remap guest addresses
for non-virt, then, rather than invent some new, arbitrary address, we
just use the host address. Remapping is still in use, but, as I said
above, it's an identity mapping.

(Note, for the current riscv iommu, "remapping" for the non-virt case just
means keeping the set of IMSICs that a device may reach limited to just
what it should be allowed to reach.)

> 
> Be specific what you mean.

I'm always happy to clarify when asked. I'm not sure what I said that
would lead to thinking remapping was disabled for the non-virt case,
but hopefully what I wrote now clarifies that it is not.

Thanks,
drew

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