[PATCH v1] iommu/riscv: Support 32-bit register accesses

[Vivian Wang]

On 6/18/26 17:51, Guo Ren wrote:
> Hi Vivian,
>
> As noted in the RISC-V IOMMU Specification, Chapter 6:
>> Whether an 8-byte access to an IOMMU register is single-copy atomic is UNSPECIFIED, and such an access may appear, internally to the IOMMU, as if two separate 4-byte accesses — first to the high half and second to the low half — were performed.
> Therefore, the atomicity of 64-bit MMIO accesses is UNSPECIFIED and
> not clearly defined in the current ratified RISC-V IOMMU
> specification. To handle this correctly, the Linux RISC-V IOMMU driver
> should fall back to 32-bit MMIO accesses when reading 64-bit registers
> (e.g., performance counters). The behavior of 32-bit MMIO accesses is
> more precisely defined in the RISC-V IOMMU specification.
>
> Thus, many hardware vendors implement 32-bit MMIO (rather than 64-bit
> MMIO) based on the current ratified RISC-V IOMMU specification, and
> this driver does not appear to benefit from 64-bit MMIO access either.
> Performance is fundamentally constrained by bus latency; assuming that
> simply reducing the number of accesses will improve performance is an
> oversimplification that ignores the underlying hardware
> characteristics.

Just as a clarification, I had never said that the driver *should* use
64-bit accesses. In fact, I said:

    I agree that the driver should be build with 32-bit accesses to support
    these hardware.

Perhaps I wasn't clear enough. I meant that I agree that the RISC-V
IOMMU driver should use 32-bit accesses. At *no* point had I claimed
that 64-bit accesses are better.

> Hi Zhang,
>
> The hardware targeted by the 32-bit MMIO access in your patch is
> precisely the kind that deserves the official ACPI HID RSCV0004 and
> Devicetree compatible string (riscv,iommu). It is rather the hardware
> that supports only 64-bit MMIO access — whose atomicity is explicitly
> [...]

IOMMU Hardware that supports only 64-bit MMIO access for 8-byte
registers has never come up in this discussion. If they ever show up
we'll talk about them then.

Vivian "dramforever" Wang