[RFC v7 02/25] powerpc: track allocation status of all pkeys

Ram Pai linuxram at us.ibm.com
Thu Oct 19 09:04:36 AEDT 2017


On Wed, Oct 18, 2017 at 06:08:46PM +0200, Laurent Dufour wrote:
> Hi Ram,
> 
> On 31/07/2017 02:12, Ram Pai wrote:
> > Total 32 keys are available on power7 and above. However
> > pkey 0,1 are reserved. So effectively we  have  30 pkeys.
> > 
> > On 4K kernels, we do not  have  5  bits  in  the  PTE to
> > represent  all the keys; we only have 3bits.Two of those
> > keys are reserved; pkey 0 and pkey 1. So effectively  we
> > have 6 pkeys.
> 
> IIUC, the pkey 0 and 1 are reserved by the hardware, and the kernel PTE has
> only 5 bits to keep track of the pkey. Why hw pkey 0 and 1 has to be
> represented in the kernel PTE ?


Key 0 is the default key. It is reserved in the sense, it cannot
be allocated or freed. but its there and will be used as the default.
So when no key is associated with a pte, it is infact using key 0.

Good question for the hardware designers. :)

key 1 is suggested not be used because it can be used by hypervisor
(powervm) or something else. Since linux kernel does not use it,
we dont program the key in the pte.



> 
> > This patch keeps track of reserved keys, allocated  keys
> > and keys that are currently free.
> > 
> > Also it  adds  skeletal  functions  and macros, that the
> > architecture-independent code expects to be available.
> > 
> > Signed-off-by: Ram Pai <linuxram at us.ibm.com>
> > ---
> >  arch/powerpc/include/asm/book3s/64/mmu.h |    9 +++
> >  arch/powerpc/include/asm/mmu_context.h   |    1 +
> >  arch/powerpc/include/asm/pkeys.h         |   98 ++++++++++++++++++++++++++++-
> >  arch/powerpc/mm/mmu_context_book3s64.c   |    2 +
> >  arch/powerpc/mm/pkeys.c                  |    2 +
> >  5 files changed, 108 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
> > index 77529a3..104ad72 100644
> > --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> > +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> > @@ -108,6 +108,15 @@ struct patb_entry {
> >  #ifdef CONFIG_SPAPR_TCE_IOMMU
> >  	struct list_head iommu_group_mem_list;
> >  #endif
> > +
> > +#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> > +	/*
> > +	 * Each bit represents one protection key.
> > +	 * bit set   -> key allocated
> > +	 * bit unset -> key available for allocation
> > +	 */
> > +	u32 pkey_allocation_map;
> > +#endif
> >  } mm_context_t;
> > 
> >  /*
> > diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> > index 4b93547..4705dab 100644
> > --- a/arch/powerpc/include/asm/mmu_context.h
> > +++ b/arch/powerpc/include/asm/mmu_context.h
> > @@ -184,6 +184,7 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
> > 
> >  #ifndef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> >  #define pkey_initialize()
> > +#define pkey_mm_init(mm)
> >  #endif /* CONFIG_PPC64_MEMORY_PROTECTION_KEYS */
> > 
> >  #endif /* __KERNEL__ */
> > diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
> > index 4ccb8f5..def385f 100644
> > --- a/arch/powerpc/include/asm/pkeys.h
> > +++ b/arch/powerpc/include/asm/pkeys.h
> > @@ -2,6 +2,8 @@
> >  #define _ASM_PPC64_PKEYS_H
> > 
> >  extern bool pkey_inited;
> > +extern int pkeys_total; /* total pkeys as per device tree */
> > +extern u32 initial_allocation_mask;/* bits set for reserved keys */
> > 
> >  /*
> >   * powerpc needs an additional vma bit to support 32 keys.
> > @@ -20,21 +22,76 @@
> >  #define VM_PKEY_BIT4	VM_HIGH_ARCH_4
> >  #endif
> > 
> > -#define ARCH_VM_PKEY_FLAGS 0
> > +#define arch_max_pkey()  pkeys_total
> > +#define ARCH_VM_PKEY_FLAGS (VM_PKEY_BIT0 | VM_PKEY_BIT1 | VM_PKEY_BIT2 | \
> > +				VM_PKEY_BIT3 | VM_PKEY_BIT4)
> > +
> > +#define pkey_alloc_mask(pkey) (0x1 << pkey)
> > +
> > +#define mm_pkey_allocation_map(mm)	(mm->context.pkey_allocation_map)
> > +
> > +#define mm_set_pkey_allocated(mm, pkey) {	\
> > +	mm_pkey_allocation_map(mm) |= pkey_alloc_mask(pkey); \
> > +}
> > +
> > +#define mm_set_pkey_free(mm, pkey) {	\
> > +	mm_pkey_allocation_map(mm) &= ~pkey_alloc_mask(pkey);	\
> > +}
> > +
> > +#define mm_set_pkey_is_allocated(mm, pkey)	\
> > +	(mm_pkey_allocation_map(mm) & pkey_alloc_mask(pkey))
> > +
> > +#define mm_set_pkey_is_reserved(mm, pkey) (initial_allocation_mask & \
> > +					pkey_alloc_mask(pkey))
> 
> This macro doesn't need a 'mm' argument.
> 
> >  static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
> >  {
> > -	return (pkey == 0);
> > +	/* a reserved key is never considered as 'explicitly allocated' */
> > +	return ((pkey < arch_max_pkey()) &&
> > +		!mm_set_pkey_is_reserved(mm, pkey) &&
> > +		mm_set_pkey_is_allocated(mm, pkey));
> >  }
> > 
> > +/*
> > + * Returns a positive, 5-bit key on success, or -1 on failure.
> 
> I guess you rely on the mmap_sem to protect against concurrency in
> mm_pkey_alloc() and mm_pkey_free().
> As this is not explicit in the code, it should at least be mentioned in the
> comment describing the function.


Yes. will do. good point.

> 
> > + */
> >  static inline int mm_pkey_alloc(struct mm_struct *mm)
> >  {
> > -	return -1;
> > +	/*
> > +	 * Note: this is the one and only place we make sure
> > +	 * that the pkey is valid as far as the hardware is
> > +	 * concerned.  The rest of the kernel trusts that
> > +	 * only good, valid pkeys come out of here.
> > +	 */
> > +	u32 all_pkeys_mask = (u32)(~(0x0));
> > +	int ret;
> > +
> > +	if (!pkey_inited)
> > +		return -1;
> > +	/*
> > +	 * Are we out of pkeys?  We must handle this specially
> > +	 * because ffz() behavior is undefined if there are no
> > +	 * zeros.
> > +	 */
> > +	if (mm_pkey_allocation_map(mm) == all_pkeys_mask)
> > +		return -1;
> > +
> > +	ret = ffz((u32)mm_pkey_allocation_map(mm));
> > +	mm_set_pkey_allocated(mm, ret);
> > +	return ret;
> >  }
> > 
> >  static inline int mm_pkey_free(struct mm_struct *mm, int pkey)
> >  {
> > -	return -EINVAL;
> > +	if (!pkey_inited)
> > +		return -1;
> > +
> > +	if (!mm_pkey_is_allocated(mm, pkey))
> > +		return -EINVAL;
> > +
> > +	mm_set_pkey_free(mm, pkey);
> > +
> > +	return 0;
> >  }
> > 
> >  /*
> > @@ -58,12 +115,45 @@ static inline int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
> >  	return 0;
> >  }
> > 
> > +static inline void pkey_mm_init(struct mm_struct *mm)
> > +{
> > +	if (!pkey_inited)
> > +		return;
> > +	mm_pkey_allocation_map(mm) = initial_allocation_mask;
> > +}
> > +
> >  static inline void pkey_initialize(void)
> >  {
> > +	int os_reserved, i;
> > +
> >  	/* disable the pkey system till everything
> >  	 * is in place. A patch further down the
> >  	 * line will enable it.
> >  	 */
> >  	pkey_inited = false;
> > +
> > +	/* Lets assume 32 keys */
> > +	pkeys_total = 32;
> > +
> > +#ifdef CONFIG_PPC_4K_PAGES
> > +	/*
> > +	 * the OS can manage only 8 pkeys
> > +	 * due to its inability to represent
> > +	 * them in the linux 4K-PTE.
> > +	 */
> > +	os_reserved = pkeys_total-8;
> > +#else
> > +	os_reserved = 0;
> > +#endif
> > +	/*
> > +	 * Bits are in LE format.
> > +	 * NOTE: 1, 0 are reserved.
> > +	 * key 0 is the default key, which allows read/write/execute.
> > +	 * key 1 is recommended not to be used.
> > +	 * PowerISA(3.0) page 1015, programming note.
> > +	 */
> > +	initial_allocation_mask = ~0x0;
> > +	for (i = 2; i < (pkeys_total - os_reserved); i++)
> > +		initial_allocation_mask &= ~(0x1<<i);
> >  }
> >  #endif /*_ASM_PPC64_PKEYS_H */
> > diff --git a/arch/powerpc/mm/mmu_context_book3s64.c b/arch/powerpc/mm/mmu_context_book3s64.c
> > index a3edf81..34a16f3 100644
> > --- a/arch/powerpc/mm/mmu_context_book3s64.c
> > +++ b/arch/powerpc/mm/mmu_context_book3s64.c
> > @@ -16,6 +16,7 @@
> >  #include <linux/string.h>
> >  #include <linux/types.h>
> >  #include <linux/mm.h>
> > +#include <linux/pkeys.h>
> >  #include <linux/spinlock.h>
> >  #include <linux/idr.h>
> >  #include <linux/export.h>
> > @@ -120,6 +121,7 @@ static int hash__init_new_context(struct mm_struct *mm)
> > 
> >  	subpage_prot_init_new_context(mm);
> > 
> > +	pkey_mm_init(mm);
> >  	return index;
> >  }
> > 
> > diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
> > index c3acee1..37dacc5 100644
> > --- a/arch/powerpc/mm/pkeys.c
> > +++ b/arch/powerpc/mm/pkeys.c
> > @@ -16,3 +16,5 @@
> >  #include <linux/pkeys.h>                /* PKEY_*                       */
> > 
> >  bool pkey_inited;
> > +int  pkeys_total;		/* total pkeys as per device tree */
> > +u32  initial_allocation_mask;	/* bits set for reserved keys */
> > 

-- 
Ram Pai



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