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

[Guo Ren <guoren@kernel.org>]

Hi Zhanpeng Zhang,

I noticed you posted a similar fix recently. However, I had already
submitted a similar solution back in September 2025 [1]. It would be
great if you could review it. A few concerns with the current patch:

1. Using spin_lock_irqsave in such a small semantic structure is
unnecessary. IRQ protection should be provided by the higher-level
IOMMU driver interfaces. Many existing call sites already handle IRQ
disabling, so adding it here feels redundant and adds avoidable
overhead.

2. More critically, the RISCV_IOMMU_32BIT_ACCESS Kconfig option is
problematic. According to the RISC-V IOMMU specification (Software
Guidelines, “Reading and writing IOMMU registers”):

> Registers that are 64-bit wide may be accessed using either a 32-bit
> or a 64-bit access.

This clearly requires that any hardware supporting 64-bit MMIO access
must also support 32-bit MMIO access. Therefore, the IOMMU driver should
be built on a 32-bit access base for maximum hardware compatibility.

3. Only performance-monitoring counters require 64-bit IO access or the
high-low-high do-while retry strategy. For ordinary status and control
MMIO registers, a single read is sufficient.

4. Introducing a compile-time Kconfig that forces 32-bit splitting (or
64-bit access) across the entire driver is unnecessary for the vast
majority of registers and would prevent a single kernel Image (or .ko)
from working across all compliant platforms.

I believe the correct approach is to keep the driver on a 32-bit access
foundation (as required by the spec) and apply special 64-bit or
high-low-high handling only to performance-monitoring counters when
justified by data. This preserves broad hardware support without
unnecessary complexity.

What do you think? I’m happy to discuss or collaborate on a cleaner
solution based on the earlier patch [1].

[1]: https://lore.kernel.org/linux-riscv/20250903144217.837448-1-guoren@kernel.org/

Best Regards
  GUO Ren

> Some RISC-V IOMMU implementations cannot perform 64-bit MMIO accesses
> to the IOMMU register file. The RISC-V IOMMU architecture allows 64-bit
> registers to be accessed using 32-bit accesses, provided the accesses are
> properly aligned and do not span multiple registers.
> 
> Add a config option for such implementations and access 64-bit IOMMU
> registers as paired 32-bit MMIO operations when it is enabled. Serialize
> the paired accesses so the high and low halves cannot interleave with
> another CPU. Full 64-bit register programming writes the high half before
> the low half.
> 
> This option describes the register access width. It is not an RV32 kernel
> mode and does not describe a 32-bit IOMMU architecture.
> 
> Co-developed-by: Xu Lu <luxu.kernel@bytedance.com>
> Signed-off-by: Xu Lu <luxu.kernel@bytedance.com>
> Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper@bytedance.com>
> ---
> This is needed for platforms whose RISC-V IOMMU register window does not
> support naturally aligned 64-bit MMIO accesses.
> 
>  drivers/iommu/riscv/Kconfig | 11 ++++++++
>  drivers/iommu/riscv/iommu.c |  4 +++
>  drivers/iommu/riscv/iommu.h | 55 +++++++++++++++++++++++++++++++++----
>  3 files changed, 64 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iommu/riscv/Kconfig b/drivers/iommu/riscv/Kconfig
> index b86e5ab94183..54d624b9b2ef 100644
> --- a/drivers/iommu/riscv/Kconfig
> +++ b/drivers/iommu/riscv/Kconfig
> @@ -22,3 +22,14 @@ config RISCV_IOMMU_PCI
>  	def_bool y if RISCV_IOMMU && PCI_MSI
>  	help
>  	  Support for the PCIe implementation of RISC-V IOMMU architecture.
> +
> +config RISCV_IOMMU_32BIT_ACCESS
> +	bool "Use 32-bit accesses for RISC-V IOMMU registers"
> +	depends on RISCV_IOMMU
> +	help
> +	  Say Y when the RISC-V IOMMU MMIO window cannot be accessed
> +	  using naturally aligned 64-bit loads and stores.
> +
> +	  When enabled, 64-bit IOMMU registers are accessed as paired
> +	  32-bit MMIO operations. This option does not describe an RV32
> +	  kernel or a 32-bit IOMMU architecture.
> diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
> index a31f50bbad35..7fa1721b5728 100644
> --- a/drivers/iommu/riscv/iommu.c
> +++ b/drivers/iommu/riscv/iommu.c
> @@ -53,6 +53,10 @@ struct riscv_iommu_devres {
>  	void *addr;
>  };
>  
> +#ifdef CONFIG_RISCV_IOMMU_32BIT_ACCESS
> +DEFINE_RAW_SPINLOCK(riscv_iommu_32bit_access_lock);
> +#endif
> +
>  static void riscv_iommu_devres_pages_release(struct device *dev, void *res)
>  {
>  	struct riscv_iommu_devres *devres = res;
> diff --git a/drivers/iommu/riscv/iommu.h b/drivers/iommu/riscv/iommu.h
> index 46df79dd5495..ba78ef1858c5 100644
> --- a/drivers/iommu/riscv/iommu.h
> +++ b/drivers/iommu/riscv/iommu.h
> @@ -14,6 +14,9 @@
>  #include <linux/iommu.h>
>  #include <linux/types.h>
>  #include <linux/iopoll.h>
> +#ifdef CONFIG_RISCV_IOMMU_32BIT_ACCESS
> +#include <linux/spinlock.h>
> +#endif
>  
>  #include "iommu-bits.h"
>  
> @@ -69,21 +72,61 @@ void riscv_iommu_disable(struct riscv_iommu_device *iommu);
>  #define riscv_iommu_readl(iommu, addr) \
>  	readl_relaxed((iommu)->reg + (addr))
>  
> -#define riscv_iommu_readq(iommu, addr) \
> -	readq_relaxed((iommu)->reg + (addr))
> -
>  #define riscv_iommu_writel(iommu, addr, val) \
>  	writel_relaxed((val), (iommu)->reg + (addr))
>  
> +#define riscv_iommu_readl_timeout(iommu, addr, val, cond, delay_us, timeout_us) \
> +	readx_poll_timeout(readl_relaxed, (iommu)->reg + (addr), val, cond, \
> +			   delay_us, timeout_us)
> +
> +#ifndef CONFIG_RISCV_IOMMU_32BIT_ACCESS
> +#define riscv_iommu_readq(iommu, addr) \
> +	readq_relaxed((iommu)->reg + (addr))
> +
>  #define riscv_iommu_writeq(iommu, addr, val) \
>  	writeq_relaxed((val), (iommu)->reg + (addr))
>  
>  #define riscv_iommu_readq_timeout(iommu, addr, val, cond, delay_us, timeout_us) \
>  	readx_poll_timeout(readq_relaxed, (iommu)->reg + (addr), val, cond, \
>  			   delay_us, timeout_us)
> +#else /* CONFIG_RISCV_IOMMU_32BIT_ACCESS */
>  
> -#define riscv_iommu_readl_timeout(iommu, addr, val, cond, delay_us, timeout_us) \
> -	readx_poll_timeout(readl_relaxed, (iommu)->reg + (addr), val, cond, \
> -			   delay_us, timeout_us)
> +extern raw_spinlock_t riscv_iommu_32bit_access_lock;
> +
> +static inline u64 __riscv_iommu_readq_relaxed(void __iomem *addr)
> +{
> +	u32 lo, hi;
> +	unsigned long flags;
> +
> +	raw_spin_lock_irqsave(&riscv_iommu_32bit_access_lock, flags);
> +	do {
> +		hi = readl_relaxed(addr + sizeof(u32));
> +		lo = readl_relaxed(addr);
> +	} while (hi != readl_relaxed(addr + sizeof(u32)));
> +	raw_spin_unlock_irqrestore(&riscv_iommu_32bit_access_lock, flags);
> +
> +	return ((u64)hi << 32) | (u64)lo;
> +}
> +
> +static inline void __riscv_iommu_writeq_relaxed(u64 value, void __iomem *addr)
> +{
> +	unsigned long flags;
> +
> +	raw_spin_lock_irqsave(&riscv_iommu_32bit_access_lock, flags);
> +	writel_relaxed((u32)(value >> 32), addr + sizeof(u32));
> +	writel_relaxed((u32)value, addr);
> +	raw_spin_unlock_irqrestore(&riscv_iommu_32bit_access_lock, flags);
> +}
> +
> +#define riscv_iommu_readq(iommu, addr) \
> +	__riscv_iommu_readq_relaxed((iommu)->reg + (addr))
> +
> +#define riscv_iommu_writeq(iommu, addr, val) \
> +	__riscv_iommu_writeq_relaxed((val), (iommu)->reg + (addr))
> +
> +#define riscv_iommu_readq_timeout(iommu, addr, val, cond, delay_us, timeout_us) \
> +	readx_poll_timeout(__riscv_iommu_readq_relaxed, (iommu)->reg + (addr), \
> +			   val, cond, delay_us, timeout_us)
> +#endif /* CONFIG_RISCV_IOMMU_32BIT_ACCESS */
>  
>  #endif
> -- 
> 2.50.1 (Apple Git-155)