Re: [PATCH 5/7] x86/mmu: Allocate/free PASID

Thomas Gleixner
2020-04-26T14:55:49Z

Fenghua Yu <fenghua.yu@intel.com> writes:

> PASID is shared by all threads in a process. So the logical place to keep
> track of it is in the "mm". Add the field to the architecture specific
> mm_context_t structure.
>
> A PASID is allocated for an "mm" the first time any thread attaches
> to an SVM capable device. Later device atatches (whether to the same

atatches?

> 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).

I'm not buying that. If there is an outstanding request with the PASID
of a process then tearing down the process address space and freeing the
PASID (which might be reused) is fundamentally broken.

> +void __free_pasid(struct mm_struct *mm);
> +
> #endif /* _ASM_X86_IOMMU_H */
> diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
> index bdeae9291e5c..137bf51f19e6 100644
> --- a/arch/x86/include/asm/mmu.h
> +++ b/arch/x86/include/asm/mmu.h
> @@ -50,6 +50,10 @@ typedef struct {
> u16 pkey_allocation_map;
> s16 execute_only_pkey;
> #endif
> +
> +#ifdef CONFIG_INTEL_IOMMU_SVM
> + int pasid;

int? It's a value which gets programmed into the MSR along with the
valid bit (bit 31) set.

> 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 d7f2a5358900..da718a49e91e 100644
> --- a/drivers/iommu/intel-svm.c
> +++ b/drivers/iommu/intel-svm.c
> @@ -226,6 +226,45 @@ static LIST_HEAD(global_svm_list);
> list_for_each_entry((sdev), &(svm)->devs, list) \
> if ((d) != (sdev)->dev) {} else
>
> +/*
> + * If this mm already has a PASID we can use it. Otherwise allocate a new one.
> + * Let the caller know if we did an allocation via 'new_pasid'.
> + */
> +static int alloc_pasid(struct intel_svm *svm, struct mm_struct *mm,
> + int pasid_max, bool *new_pasid, int flags)

Again, data types please. flags are generally unsigned and not plain
int. Also pasid_max is certainly not plain int either.

> +{
> + int pasid;
> +
> + /*
> + * Reuse the PASID if the mm already has a PASID and not a private
> + * PASID is requested.
> + */
> + if (mm && mm->context.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->context.pasid, svm);

So if the PASID is reused several times for different SVMs then every
time ioasid_data->private is set to a different SVM. How is that
supposed to work?

> + *new_pasid = false;
> +
> + return mm->context.pasid;
> + }
> +
> + /*
> + * Allocate a new pasid. Do not use PASID 0, reserved for RID to
> + * PASID.
> + */
> + pasid = ioasid_alloc(NULL, PASID_MIN, pasid_max - 1, svm);

ioasid_alloc() uses ioasid_t which is

typedef unsigned int ioasid_t;

Can we please have consistent types and behaviour all over the place?

> + if (pasid == INVALID_IOASID)
> + return -ENOSPC;
> +
> + *new_pasid = true;
> +
> + return pasid;
> +}
> +
> int intel_svm_bind_mm(struct device *dev, int *pasid, int flags, struct svm_dev_ops *ops)
> {
> struct intel_iommu *iommu = intel_svm_device_to_iommu(dev);
> @@ -324,6 +363,8 @@ int intel_svm_bind_mm(struct device *dev, int *pasid, int flags, struct svm_dev_
> init_rcu_head(&sdev->rcu);
>
> if (!svm) {
> + bool new_pasid;
> +
> svm = kzalloc(sizeof(*svm), GFP_KERNEL);
> if (!svm) {
> ret = -ENOMEM;
> @@ -335,15 +376,13 @@ int intel_svm_bind_mm(struct device *dev, int *pasid, int flags, struct svm_dev_
> 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);
> - if (svm->pasid == INVALID_IOASID) {
> + svm->pasid = alloc_pasid(svm, mm, pasid_max, &new_pasid, flags);
> + if (svm->pasid < 0) {
> kfree(svm);
> kfree(sdev);
> - ret = -ENOSPC;

ret gets magically initialized to an error return value, right?

> goto out;
> }
> +
> svm->notifier.ops = &intel_mmuops;
> svm->mm = mm;
> svm->flags = flags;
> @@ -353,7 +392,8 @@ int intel_svm_bind_mm(struct device *dev, int *pasid, int flags, struct svm_dev_
> if (mm) {
> ret = mmu_notifier_register(&svm->notifier, mm);
> if (ret) {
> - ioasid_free(svm->pasid);
> + if (new_pasid)
> + ioasid_free(svm->pasid);
> kfree(svm);
> kfree(sdev);
> goto out;
> @@ -371,12 +411,21 @@ int intel_svm_bind_mm(struct device *dev, int *pasid, int flags, struct svm_dev_
> if (ret) {
> if (mm)
> mmu_notifier_unregister(&svm->notifier, mm);
> - ioasid_free(svm->pasid);
> + if (new_pasid)
> + ioasid_free(svm->pasid);
> kfree(svm);
> kfree(sdev);

So there are 3 places now freeing svm ad sdev and 2 of them
conditionally free svm->pasid. Can you please rewrite that to have a
proper error exit path instead of glueing that stuff into the existing
mess?

> 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.

What happens to private PASIDs?

Thanks,

tglx