Re: [PATCH v2 11/12] x86/mmu: Allocate/free PASID

[Lu Baolu]

Hi Fenghua,

On 2020/6/13 8:41, Fenghua Yu wrote:
> A PASID is allocated for an "mm" the first time any thread attaches
> to an SVM capable device. Later device attachments (whether to the same
> device or another SVM device) will re-use the same PASID.
>
> The PASID is freed when the process exits (so no need to keep
> reference counts on how many SVM devices are sharing the PASID).

FYI.

Jean-Philippe Brucker has a patch for mm->pasid management in the vendor
agnostic manner.

https://www.spinics.net/lists/iommu/msg44459.html

Best regards,
baolu

> Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---
> v2:
> - Define a helper free_bind() to simplify error exit code in bind_mm()
>    (Thomas)
> - Fix a ret error code in bind_mm() (Thomas)
> - Change pasid's type from "int" to "unsigned int" to have consistent
>    pasid type in iommu (Thomas)
> - Simplify alloc_pasid() a bit.
>
>   arch/x86/include/asm/iommu.h       |   2 +
>   arch/x86/include/asm/mmu_context.h |  14 ++++
>   drivers/iommu/intel/svm.c          | 101 +++++++++++++++++++++++++----
>   3 files changed, 105 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/iommu.h b/arch/x86/include/asm/iommu.h
> index bf1ed2ddc74b..ed41259fe7ac 100644
> --- a/arch/x86/include/asm/iommu.h
> +++ b/arch/x86/include/asm/iommu.h
> @@ -26,4 +26,6 @@ arch_rmrr_sanity_check(struct acpi_dmar_reserved_memory *rmrr)
>   	return -EINVAL;
>   }
>   
> +void __free_pasid(struct mm_struct *mm);
> +
>   #endif /* _ASM_X86_IOMMU_H */
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index 47562147e70b..f8c91ce8c451 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -13,6 +13,7 @@
>   #include <asm/tlbflush.h>
>   #include <asm/paravirt.h>
>   #include <asm/debugreg.h>
> +#include <asm/iommu.h>
>   
>   extern atomic64_t last_mm_ctx_id;
>   
> @@ -117,9 +118,22 @@ static inline int init_new_context(struct task_struct *tsk,
>   	init_new_context_ldt(mm);
>   	return 0;
>   }
> +
> +static inline void free_pasid(struct mm_struct *mm)
> +{
> +	if (!IS_ENABLED(CONFIG_INTEL_IOMMU_SVM))
> +		return;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_ENQCMD))
> +		return;
> +
> +	__free_pasid(mm);
> +}
> +
>   static inline void destroy_context(struct mm_struct *mm)
>   {
>   	destroy_context_ldt(mm);
> +	free_pasid(mm);
>   }
>   
>   extern void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
> index 4e775e12ae52..27dc866b8461 100644
> --- a/drivers/iommu/intel/svm.c
> +++ b/drivers/iommu/intel/svm.c
> @@ -425,6 +425,53 @@ int intel_svm_unbind_gpasid(struct device *dev, unsigned int pasid)
>   	return ret;
>   }
>   
> +static void free_bind(struct intel_svm *svm, struct intel_svm_dev *sdev,
> +		      bool new_pasid)
> +{
> +	if (new_pasid)
> +		ioasid_free(svm->pasid);
> +	kfree(svm);
> +	kfree(sdev);
> +}
> +
> +/*
> + * If this mm already has a PASID, use it. Otherwise allocate a new one.
> + * Let the caller know if a new PASID is allocated via 'new_pasid'.
> + */
> +static int alloc_pasid(struct intel_svm *svm, struct mm_struct *mm,
> +		       unsigned int pasid_max, bool *new_pasid,
> +		       unsigned int flags)
> +{
> +	unsigned int pasid;
> +
> +	*new_pasid = false;
> +
> +	/*
> +	 * Reuse the PASID if the mm already has a PASID and not a private
> +	 * PASID is requested.
> +	 */
> +	if (mm && mm->pasid && !(flags & SVM_FLAG_PRIVATE_PASID)) {
> +		/*
> +		 * Once a PASID is allocated for this mm, the PASID
> +		 * stays with the mm until the mm is dropped. Reuse
> +		 * the PASID which has been already allocated for the
> +		 * mm instead of allocating a new one.
> +		 */
> +		ioasid_set_data(mm->pasid, svm);
> +
> +		return mm->pasid;
> +	}
> +
> +	/* Allocate a new pasid. Do not use PASID 0, reserved for init PASID. */
> +	pasid = ioasid_alloc(NULL, PASID_MIN, pasid_max - 1, svm);
> +	if (pasid != INVALID_IOASID) {
> +		/* A new pasid is allocated. */
> +		*new_pasid = true;
> +	}
> +
> +	return pasid;
> +}
> +
>   /* Caller must hold pasid_mutex, mm reference */
>   static int
>   intel_svm_bind_mm(struct device *dev, unsigned int flags,
> @@ -518,6 +565,8 @@ intel_svm_bind_mm(struct device *dev, unsigned int flags,
>   	init_rcu_head(&sdev->rcu);
>   
>   	if (!svm) {
> +		bool new_pasid;
> +
>   		svm = kzalloc(sizeof(*svm), GFP_KERNEL);
>   		if (!svm) {
>   			ret = -ENOMEM;
> @@ -529,12 +578,9 @@ intel_svm_bind_mm(struct device *dev, unsigned int flags,
>   		if (pasid_max > intel_pasid_max_id)
>   			pasid_max = intel_pasid_max_id;
>   
> -		/* Do not use PASID 0, reserved for RID to PASID */
> -		svm->pasid = ioasid_alloc(NULL, PASID_MIN,
> -					  pasid_max - 1, svm);
> +		svm->pasid = alloc_pasid(svm, mm, pasid_max, &new_pasid, flags);
>   		if (svm->pasid == INVALID_IOASID) {
> -			kfree(svm);
> -			kfree(sdev);
> +			free_bind(svm, sdev, new_pasid);
>   			ret = -ENOSPC;
>   			goto out;
>   		}
> @@ -547,9 +593,7 @@ intel_svm_bind_mm(struct device *dev, unsigned int flags,
>   		if (mm) {
>   			ret = mmu_notifier_register(&svm->notifier, mm);
>   			if (ret) {
> -				ioasid_free(svm->pasid);
> -				kfree(svm);
> -				kfree(sdev);
> +				free_bind(svm, sdev, new_pasid);
>   				goto out;
>   			}
>   		}
> @@ -565,12 +609,18 @@ intel_svm_bind_mm(struct device *dev, unsigned int flags,
>   		if (ret) {
>   			if (mm)
>   				mmu_notifier_unregister(&svm->notifier, mm);
> -			ioasid_free(svm->pasid);
> -			kfree(svm);
> -			kfree(sdev);
> +			free_bind(svm, sdev, new_pasid);
>   			goto out;
>   		}
>   
> +		if (mm && new_pasid && !(flags & SVM_FLAG_PRIVATE_PASID)) {
> +			/*
> +			 * Track the new pasid in the mm. The pasid will be
> +			 * freed at process exit. Don't track requested
> +			 * private PASID in the mm.
> +			 */
> +			mm->pasid = svm->pasid;
> +		}
>   		list_add_tail(&svm->list, &global_svm_list);
>   	} else {
>   		/*
> @@ -640,7 +690,8 @@ static int intel_svm_unbind_mm(struct device *dev, unsigned int pasid)
>   			kfree_rcu(sdev, rcu);
>   
>   			if (list_empty(&svm->devs)) {
> -				ioasid_free(svm->pasid);
> +				/* Clear data in the pasid. */
> +				ioasid_set_data(pasid, NULL);
>   				if (svm->mm)
>   					mmu_notifier_unregister(&svm->notifier, svm->mm);
>   				list_del(&svm->list);
> @@ -1001,3 +1052,29 @@ unsigned int intel_svm_get_pasid(struct iommu_sva *sva)
>   
>   	return pasid;
>   }
> +
> +/*
> + * An invalid pasid is either 0 (init PASID value) or bigger than max PASID
> + * (PASID_MAX - 1).
> + */
> +static bool invalid_pasid(unsigned int pasid)
> +{
> +	return (pasid == INIT_PASID) || (pasid >= PASID_MAX);
> +}
> +
> +/* On process exit free the PASID (if one was allocated). */
> +void __free_pasid(struct mm_struct *mm)
> +{
> +	unsigned int pasid = mm->pasid;
> +
> +	/* No need to free invalid pasid. */
> +	if (invalid_pasid(pasid))
> +		return;
> +
> +	/*
> +	 * Since the pasid is not bound to any svm by now, there is no race
> +	 * here with binding/unbinding and no need to protect the free
> +	 * operation by pasid_mutex.
> +	 */
> +	ioasid_free(pasid);
> +}