Re: [PATCH RFC 10/11] iommu: Make IOPF handling framework generic

[Lu Baolu]

On 2022/3/21 20:50, Jason Gunthorpe wrote:
> On Sun, Mar 20, 2022 at 02:40:29PM +0800, Lu Baolu wrote:
>
> > +static enum iommu_page_response_code
> > +iommu_sva_handle_iopf(struct iommu_fault *fault, void *data)
> > +{
> > +	vm_fault_t ret;
> > +	struct mm_struct *mm;
> > +	struct vm_area_struct *vma;
> > +	unsigned int access_flags = 0;
> > +	struct iommu_domain *domain = data;
>
> Why is the iommu_domain not passed in as a fully typed object? I would
> think data should some opaque value used by non-sva cases.

The "data" is set together with the fault handler when the caller
installs a fault handler for an iommu domain. We will add a generic
interface to install fault handler for an iommu domain later when a real
non-sva case comes.

> What is the lifetime model here anyhow?

I simply thought that the device driver should guarantee that there are
no pending faults after sva_unbind(). This is insufficient. I need to
rework this.

> > +	unsigned int fault_flags = FAULT_FLAG_REMOTE;
> > +	struct iommu_fault_page_request *prm = &fault->prm;
> > +	enum iommu_page_response_code status = IOMMU_PAGE_RESP_INVALID;
> > +
> > +	if (!(prm->flags & IOMMU_FAULT_PAGE_REQUEST_PASID_VALID))
> > +		return status;
> > +
> > +	mm = domain->sva_cookie;
> > +	if (IS_ERR_OR_NULL(mm))
>
> Do not use this function
>
> Do not store err pointers in structs.

Sure.

> > +out_put_mm:
> > +	mmap_read_unlock(mm);
> > +	mmput(mm);
>
> mm structs are weird, they have two refcounts.
>
> The 'sva_cookie' should hold a mmgrab/mmdrop() refcount to keep the
> pointer alive but to touch the mmap lock you have to upgrade it to a
> refcount that prevents destruction using mmget_not_zero() just for
> this short period.

Yes. Will look into it.

Best regards,
baolu