linux-kernel - Re: [PATCH v7 2/5] iommu: Add return value rules to attach

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Date:   Tue, 18 Oct 2022 09:20:52 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Nicolin Chen <nicolinc@...dia.com>, joro@...tes.org,
        will@...nel.org, robin.murphy@....com
Cc:     baolu.lu@...ux.intel.com, jgg@...dia.com, kevin.tian@...el.com,
        quic_jjohnson@...cinc.com, suravee.suthikulpanit@....com,
        robdclark@...il.com, dwmw2@...radead.org, yong.wu@...iatek.com,
        matthias.bgg@...il.com, orsonzhai@...il.com,
        baolin.wang@...ux.alibaba.com, zhang.lyra@...il.com,
        thierry.reding@...il.com, vdumpa@...dia.com, jonathanh@...dia.com,
        jean-philippe@...aro.org, tglx@...utronix.de,
        shameerali.kolothum.thodi@...wei.com,
        christophe.jaillet@...adoo.fr, yangyicong@...ilicon.com,
        yangyingliang@...wei.com, quic_saipraka@...cinc.com,
        jon@...id-run.com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-tegra@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v7 2/5] iommu: Add return value rules to attach_dev op and
 APIs

On 2022/10/18 7:01, Nicolin Chen wrote:
> Cases like VFIO wish to attach a device to an existing domain that was
> not allocated specifically from the device. This raises a condition
> where the IOMMU driver can fail the domain attach because the domain and
> device are incompatible with each other.
> 
> This is a soft failure that can be resolved by using a different domain.
> 
> Provide a dedicated errno EINVAL from the IOMMU driver during attach that
> the reason why the attach failed is because of domain incompatibility.
> 
> VFIO can use this to know that the attach is a soft failure and it should
> continue searching. Otherwise, the attach will be a hard failure and VFIO
> will return the code to userspace.
> 
> Update kdocs to add rules of return value to the attach_dev op and APIs.
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Reviewed-by: Kevin Tian <kevin.tian@...el.com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>   drivers/iommu/iommu.c | 24 ++++++++++++++++++++++++
>   include/linux/iommu.h | 12 ++++++++++++
>   2 files changed, 36 insertions(+)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 4893c2429ca5..3c0c5d64bb50 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -1949,6 +1949,18 @@ static int __iommu_attach_device(struct iommu_domain *domain,
>   	return ret;
>   }
>   
> +/**
> + * iommu_attach_device - Attach an IOMMU domain to a device
> + * @domain: IOMMU domain to attach
> + * @dev: Device that will be attached
> + *
> + * Returns 0 on success and error code on failure
> + *
> + * Note that EINVAL can be treated as a soft failure, indicating
> + * that certain configuration of the domain is incompatible with
> + * the device. In this case attaching a different domain to the
> + * device may succeed.
> + */
>   int iommu_attach_device(struct iommu_domain *domain, struct device *dev)
>   {
>   	struct iommu_group *group;
> @@ -2075,6 +2087,18 @@ static int __iommu_attach_group(struct iommu_domain *domain,
>   	return ret;
>   }
>   
> +/**
> + * iommu_attach_group - Attach an IOMMU domain to an IOMMU group
> + * @domain: IOMMU domain to attach
> + * @group: IOMMU group that will be attached
> + *
> + * Returns 0 on success and error code on failure
> + *
> + * Note that EINVAL can be treated as a soft failure, indicating
> + * that certain configuration of the domain is incompatible with
> + * the group. In this case attaching a different domain to the
> + * group may succeed.
> + */
>   int iommu_attach_group(struct iommu_domain *domain, struct iommu_group *group)
>   {
>   	int ret;
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index a325532aeab5..6f3569340e8a 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -266,6 +266,18 @@ struct iommu_ops {
>   /**
>    * struct iommu_domain_ops - domain specific operations
>    * @attach_dev: attach an iommu domain to a device
> + *  Return:
> + * * 0		- success
> + * * EINVAL	- can indicate that device and domain are incompatible due to
> + *		  some previous configuration of the domain, in which case the
> + *		  driver shouldn't log an error, since it is legitimate for a
> + *		  caller to test reuse of existing domains. Otherwise, it may
> + *		  still represent some other fundamental problem
> + * * ENOMEM	- out of memory
> + * * ENOSPC	- non-ENOMEM type of resource allocation failures
> + * * EBUSY	- device is attached to a domain and cannot be changed
> + * * ENODEV	- device specific errors, not able to be attached
> + * * <others>	- treated as ENODEV by the caller. Use is discouraged
>    * @detach_dev: detach an iommu domain from a device
>    * @map: map a physically contiguous memory region to an iommu domain
>    * @map_pages: map a physically contiguous set of pages of the same size to

Reviewed-by: Lu Baolu <baolu.lu@...ux.intel.com>

Best regards,
baolu