Re: [RFC PATCH v4 08/10] iommu/riscv: Pre-enable GADE for second-stage domains

[Gong Shuai]

Hi Fangyu,

On 9/15/2026 11:28 AM, fangyu.yu@xxxxxxxxxxxxxxxxx wrote:
> From: Fangyu Yu <fangyu.yu@xxxxxxxxxxxxxxxxx>
>
> Pre-enable RISCV_IOMMU_DC_TC_GADE in the device context when
> attaching a second-stage domain, if the IOMMU supports AMO_HWAD.
>
> Software pre-populates second-stage page tables with D set, so
> enabling GADE by default does not change normal behavior. When
> dirty tracking is enabled, iommufd clears the pre-set D bits and
> GADE becomes necessary for hardware to update the dirty bit on
> write access.
>
> This avoids toggling GADE dynamically and keeps device context
> setup consistent with second-stage domain attachment.
>
> Signed-off-by: Fangyu Yu <fangyu.yu@xxxxxxxxxxxxxxxxx>
> ---
>   drivers/iommu/riscv/iommu.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
> index 16779877351b..f01fd4e2f840 100644
> --- a/drivers/iommu/riscv/iommu.c
> +++ b/drivers/iommu/riscv/iommu.c
> @@ -1330,6 +1330,8 @@ static int riscv_iommu_attach_paging_domain(struct iommu_domain *iommu_domain,
>   			FIELD_PREP(RISCV_IOMMU_DC_IOHGATP_MODE, pt_info.iohgatp_mode) |
>   			FIELD_PREP(RISCV_IOMMU_DC_IOHGATP_GSCID, domain->gscid) |
>   			FIELD_PREP(RISCV_IOMMU_DC_IOHGATP_PPN, pt_info.ppn);
> +		if (iommu->caps & RISCV_IOMMU_CAPABILITIES_AMO_HWAD)
> +			dc.tc |= RISCV_IOMMU_DC_TC_GADE;

I might be missing something, but I don't see how the GADE bit set
here reaches the hardware IOMMU DC. In riscv_iommu_iodir_update()
(in patch 04/10), it looks like only the V bit is taken:

	tc = READ_ONCE(dc->tc);
	tc |= new_dc->ta & RISCV_IOMMU_DC_TC_V;
	...
	WRITE_ONCE(dc->tc, tc);

I saw that you mentioned you tested it on QEMU. As far as I know,
mainline QEMU has not yet declared the AMO_HWAD capability. Could
you clarify whether you used a modified QEMU for this test?

Thanks,
Shuai

>   	} else {
>   		if (!riscv_iommu_fsc_supported(iommu, pt_info.fsc_iosatp_mode))
>   			return -ENODEV;