Re: [PATCH v6 01/25] mm: Add a PASID field to mm_struct

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Mon, May 04, 2020 at 09:52:44AM +0800, Xu Zaibo wrote:
> 
> On 2020/4/30 22:34, Jean-Philippe Brucker wrote:
> > Some devices can tag their DMA requests with a 20-bit Process Address
> > Space ID (PASID), allowing them to access multiple address spaces. In
> > combination with recoverable I/O page faults (for example PCIe PRI),
> > PASID allows the IOMMU to share page tables with the MMU.
> > 
> > To make sure that a single PASID is allocated for each address space, as
> > required by Intel ENQCMD, store the PASID in the mm_struct. The IOMMU
> > driver is in charge of serializing modifications to the PASID field.
> > 
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > ---
> > For the field's validity I'm thinking invalid PASID = 0. In ioasid.h we
> > define INVALID_IOASID as ~0U, but I think we can now change it to 0,
> > since Intel is now also reserving PASID #0 for Transactions without
> > PASID and AMD IOMMU uses GIoV for this too.
> > ---
> >   include/linux/mm_types.h | 4 ++++
> >   1 file changed, 4 insertions(+)
> > 
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 4aba6c0c2ba80..8db6472758175 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -534,6 +534,10 @@ struct mm_struct {
> >   		atomic_long_t hugetlb_usage;
> >   #endif
> >   		struct work_struct async_put_work;
> > +#ifdef CONFIG_IOMMU_SUPPORT
> > +		/* Address space ID used by device DMA */
> > +		unsigned int pasid;
> > +#endif
> Maybe '#ifdef CONFIG_IOMMU_SVA ... #endif' is more reasonable?

CONFIG_IOMMU_SVA enables a few helpers but IOMMU drivers don't have to use
them, so I think IOMMU_SUPPORT is more appropriate.

Thanks,
Jean
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