linux-kernel - Re: [RFC PATCH 04/15] iommu/riscv: report iommu capabilities

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Message-ID: <ddd40bc3-7f2a-43c2-8918-a10c63bd05ba@arm.com>
Date: Fri, 15 Nov 2024 15:20:36 +0000
From: Robin Murphy <robin.murphy@....com>
To: Andrew Jones <ajones@...tanamicro.com>, iommu@...ts.linux.dev,
 kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc: tjeznach@...osinc.com, zong.li@...ive.com, joro@...tes.org,
 will@...nel.org, anup@...infault.org, atishp@...shpatra.org,
 tglx@...utronix.de, alex.williamson@...hat.com, paul.walmsley@...ive.com,
 palmer@...belt.com, aou@...s.berkeley.edu
Subject: Re: [RFC PATCH 04/15] iommu/riscv: report iommu capabilities

On 14/11/2024 4:18 pm, Andrew Jones wrote:
> From: Tomasz Jeznach <tjeznach@...osinc.com>
> 
> Report RISC-V IOMMU capabilities required by VFIO subsystem
> to enable PCIe device assignment.

IOMMU_CAP_DEFERRED_FLUSH has nothing at all to do with VFIO. As far as I 
can tell from what's queued, riscv_iommu_unmap_pages() isn't really 
implementing the full optimisation to get the most out of it either.

I guess IOMMU_CAP_CACHE_COHERENCY falls out of the assumption of a 
coherent IOMMU and lack of PBMT support making everything implicitly 
IOMMU_CACHE all the time whether you want it or not, but clarifying that 
might be nice (especially since there's some chance that something will 
eventually come along to break it...)

Thanks,
Robin.

> Signed-off-by: Tomasz Jeznach <tjeznach@...osinc.com>
> Signed-off-by: Andrew Jones <ajones@...tanamicro.com>
> ---
>   drivers/iommu/riscv/iommu.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
> index 8a05def774bd..3fe4ceba8dd3 100644
> --- a/drivers/iommu/riscv/iommu.c
> +++ b/drivers/iommu/riscv/iommu.c
> @@ -1462,6 +1462,17 @@ static struct iommu_group *riscv_iommu_device_group(struct device *dev)
>   	return generic_device_group(dev);
>   }
>   
> +static bool riscv_iommu_capable(struct device *dev, enum iommu_cap cap)
> +{
> +	switch (cap) {
> +	case IOMMU_CAP_CACHE_COHERENCY:
> +	case IOMMU_CAP_DEFERRED_FLUSH:
> +		return true;
> +	default:
> +		return false;
> +	}
> +}
> +
>   static int riscv_iommu_of_xlate(struct device *dev, const struct of_phandle_args *args)
>   {
>   	return iommu_fwspec_add_ids(dev, args->args, 1);
> @@ -1526,6 +1537,7 @@ static void riscv_iommu_release_device(struct device *dev)
>   static const struct iommu_ops riscv_iommu_ops = {
>   	.pgsize_bitmap = SZ_4K,
>   	.of_xlate = riscv_iommu_of_xlate,
> +	.capable = riscv_iommu_capable,
>   	.identity_domain = &riscv_iommu_identity_domain,
>   	.blocked_domain = &riscv_iommu_blocking_domain,
>   	.release_domain = &riscv_iommu_blocking_domain,