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

On Fri, Jun 19, 2026 at 12:02 AM David Laight
<[email protected]> wrote:
>
> On Thu, 18 Jun 2026 17:51:34 +0800
> Guo Ren <[email protected]> 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.
>
> If the bus latency is significant it is almost certainly worth using
> memory accesses to avoid re-reading the hi register.
>
> Something like this might work:
>
> static volatile u32 hi_prev, lo_prev;
>
>         u32 hi = read_reg_hi();
>         u32 lo = read_reg_lo();
>
>         if (lo <= lo_prev || hi != hi_prev) {
>                 u32 hi_tmp = read_reg_hi;
>                 if (hi_tmp != hi) {
>                         hi = hi_tmp;
>                         lo = 0;
>                 }
>                 lo_prev = ~0u;
>                 hi_prev = hi;
>         }
>         lo_prev = lo;
>         return (u64)hi << 32 | lo;
>

Hi Daivd,

I included this in my v5 of the IOMMU PMU series, but we noticed that
sashiko-bot AI reported two issues:

1: The lo_prev is immediately overwritten:
The trailing 'pmu->lo_prev[idx] = lo;' is outside the if block and
runs unconditionally, so the 'lo_prev = ~0u' is clobbered before the
function even returns. It never survives to the next call. lo_prev
ends up holding the guessed value (typically 0) instead, which is a
perfectly ordinary small number, and the intended "always re-verify
next time" behaviour never happens. The sentinel write was effectively
dead code.

2: The wrap check itself can miss a wrap
Here is a concrete sequence. Assume a previous call left hi_prev = 1
and lo_prev = 5, i.e. the counter was 0x1_00000005. Some time later
the counter has advanced close to 0x1_FFFFFFFF:

  1. hi = readl(addr + 4)  -> counter is 0x1_FFFFFFF0, so hi = 1
  2. the counter crosses the boundary and becomes 0x2_00000008
  3. lo = readl(addr)      -> lo = 8

The check then evaluates:

  lo (8) <= lo_prev (5)   -> false
  hi (1) != hi_prev (1)   -> false

Both are false, so the fast path is taken and 0x1_00000008 is
returned. The true value is 0x2_00000008, so the result is short by
2^32 and the third read of the high half never even executes.

Both situations share the same root cause: It puts the reliable check
(the third read) inside an if guarded by the unreliable cross-call
heuristic 'lo <= lo_prev || hi != hi_prev)'
Case 1 is an issue in the bookkeeping that heuristic depends on, and
case 2 shows that even with correct bookkeeping the heuristic is not
sound.

To make everything simpler, I would use your first version for the
latest IOMMU PMU series:

        hi = read_hi();
        lo = read_lo();
        if (hi != read_hi()) {
                // Pick a value that happened while doing the reads.
                hi++;
                lo = 0;
        }

> It shouldn't need any locking but the accesses do need to be ordered.
>
>         David
>
>
>
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