Messages in this thread | | | Date | Fri, 4 Feb 2022 16:25:34 -0800 | From | Fenghua Yu <> | Subject | Re: [PATCH v3 04/11] kernel/fork: Initialize mm's PASID |
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Hi, Thomas,
On Sat, Feb 05, 2022 at 12:22:12AM +0100, Thomas Gleixner wrote: > On Fri, Jan 28 2022 at 12:28, Fenghua Yu wrote: > > A new mm doesn't have a PASID yet when it's created. Initialize > > the mm's PASID on fork() or for init_mm to INVALID_IOASID (-1). > > I must be missing something here. > > > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > > index aa5f09ca5bcf..c74d1edbac2f 100644 > > --- a/include/linux/sched/mm.h > > +++ b/include/linux/sched/mm.h > > @@ -8,6 +8,7 @@ > > #include <linux/mm_types.h> > > #include <linux/gfp.h> > > #include <linux/sync_core.h> > > +#include <linux/ioasid.h> > > > > /* > > * Routines for handling mm_structs > > @@ -433,4 +434,13 @@ static inline void membarrier_update_current_mm(struct mm_struct *next_mm) > > } > > #endif > > > > +#ifdef CONFIG_IOMMU_SVA > > +static inline void mm_pasid_init(struct mm_struct *mm) > > +{ > > + mm->pasid = INVALID_IOASID; > > +} > > +#else > > +static inline void mm_pasid_init(struct mm_struct *mm) {} > > +#endif > > + > > #endif /* _LINUX_SCHED_MM_H */ > > So this adds mm_pasid_init() to linux/sched/mm.h which replaces: > > > -static void mm_init_pasid(struct mm_struct *mm) > > -{ > > -#ifdef CONFIG_IOMMU_SVA > > - mm->pasid = INIT_PASID; > > -#endif > > -} > > - > > I.e. already existing code which is initializing mm->pasid with > INIT_PASID (0) while the replacement initializes it with INVALID_IOASID > (-1). > > The change log does not have any information about why INIT_PASID is the > wrong initialization value and why this change is not having any side > effects.
I should add the following info in the commit message to explain why change INIT_PASID (0) to INVALID_IOASID (-1):
INIT_PASID (0) is reserved for kernel legacy DMA PASID. It cannot be allocated to a user process. Initialize the process's PASID to 0 may cause confusion that why the process uses reserved kernel legacy DMA PASID. Initializing the PASID to INVALID_IOASID (-1) explicitly tells the process doesn't have a valid PASID yet initially.
Is it OK for you?
> > It neither mentions why having this in a global available header makes > sense when the only call site is in the C file from which the already > existing function is removed.
This series defines three helpers mm_pasid_init(), mm_pasid_set(), and mm_pasid_drop() in mm because they handle the pasid member in mm_struct and should be part of mm operations. I explained why mm_pasid_set() and mm_pasid_drop() are defined in mm, but I didn't explain why mm_pasid_init() is define in mm.
Is it OK to add the following explanation on why mm_pasid_init() is defined?
mm_pasid_init() is defined in mm and replaces mm_init_pasid() because the PASID init operation initializes the pasid member in mm_struct and should be part of mm operations.
Thanks,
-Fenghua
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