Re: [PATCH v2 3/3] arm64: use generic MMIO accessors

[Qingfang Deng]

Hi,

On 2026/9/20 16:08, Arnd Bergmann wrote:
> On Sun, Sep 20, 2026, at 03:43, Qingfang Deng wrote:
> > Use the generic raw write accessors in place of the inline assembly
> > stores. Include asm-generic/mmio.h after the arm64 ordering hooks so
> > these accessors are also available to the write-combining helpers
> > before asm-generic/io.h is included.
> >
> > Retain the raw read overrides when either the Cortex-A57 erratum 832075
> > or NVIDIA Olympus erratum 1027 workaround is configured. This preserves
> > the alternative-patched load-acquire instructions and pre-load barriers.
> > Use generic raw reads when neither workaround is enabled.
> >
> > Assisted-by: Codex:gpt-6-astra
> > Signed-off-by: Qingfang Deng <qingfang.deng@xxxxxxxxx>
> Sorry, this is no good, we need the compiler to issue the exact
> instructions from the inline asm here, both in order to guarantee
> these can be virtualized, and to prevent it from splitting up
> larger register accesses into byte load/store on unaligned pointers.

You're right about virtualization: an AArch64 compiler can generate 
writeback addressing loads/stores, which KVM's MMIO handler does not 
support.

Regarding unaligned accesses, the splitting can only happen when the 
address is known to be unaligned at compile-time. Do these accessors 
need to support unaligned MMIO? If so, I could add 
OPTIMIZER_HIDE_VAR(addr) to the generic accessors.

Best regards,
Qingfang