RE: [PATCH 5/6 v2] kvm: powerpc: booke: Add linux pte lookup like booke3s

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> -----Original Message-----
> From: Bhushan Bharat-R65777
> Sent: Tuesday, August 06, 2013 6:42 AM
> To: Wood Scott-B07421
> Cc: Benjamin Herrenschmidt; agraf@xxxxxxx; kvm-ppc@xxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; linuxppc-dev@xxxxxxxxxxxxxxxx
> Subject: RE: [PATCH 5/6 v2] kvm: powerpc: booke: Add linux pte lookup like
> booke3s
> 
> 
> 
> > -----Original Message-----
> > From: Wood Scott-B07421
> > Sent: Tuesday, August 06, 2013 12:49 AM
> > To: Bhushan Bharat-R65777
> > Cc: Benjamin Herrenschmidt; Wood Scott-B07421; agraf@xxxxxxx; kvm-
> > ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> > linuxppc-dev@xxxxxxxxxxxxxxxx
> > Subject: Re: [PATCH 5/6 v2] kvm: powerpc: booke: Add linux pte lookup
> > like booke3s
> >
> > On Mon, 2013-08-05 at 09:27 -0500, Bhushan Bharat-R65777 wrote:
> > >
> > > > -----Original Message-----
> > > > From: Benjamin Herrenschmidt [mailto:benh@xxxxxxxxxxxxxxxxxxx]
> > > > Sent: Saturday, August 03, 2013 9:54 AM
> > > > To: Bhushan Bharat-R65777
> > > > Cc: Wood Scott-B07421; agraf@xxxxxxx; kvm-ppc@xxxxxxxxxxxxxxx;
> > > > kvm@xxxxxxxxxxxxxxx; linuxppc-dev@xxxxxxxxxxxxxxxx
> > > > Subject: Re: [PATCH 5/6 v2] kvm: powerpc: booke: Add linux pte
> > > > lookup like booke3s
> > > >
> > > > On Sat, 2013-08-03 at 02:58 +0000, Bhushan Bharat-R65777 wrote:
> > > > > One of the problem I saw was that if I put this code in
> > > > > asm/pgtable-32.h and asm/pgtable-64.h then pte_persent() and
> > > > > other friend function (on which this code depends) are defined in
> pgtable.h.
> > > > > And pgtable.h includes asm/pgtable-32.h and asm/pgtable-64.h
> > > > > before it defines pte_present() and friends functions.
> > > > >
> > > > > Ok I move wove this in asm/pgtable*.h, initially I fought with
> > > > > myself to take this code in pgtable* but finally end up doing
> > > > > here (got biased by book3s :)).
> > > >
> > > > Is there a reason why these routines can not be completely generic
> > > > in pgtable.h ?
> > >
> > > How about the generic function:
> > >
> > > diff --git a/arch/powerpc/include/asm/pgtable-ppc64.h
> > > b/arch/powerpc/include/asm/pgtable-ppc64.h
> > > index d257d98..21daf28 100644
> > > --- a/arch/powerpc/include/asm/pgtable-ppc64.h
> > > +++ b/arch/powerpc/include/asm/pgtable-ppc64.h
> > > @@ -221,6 +221,27 @@ static inline unsigned long pte_update(struct
> > > mm_struct
> > *mm,
> > >         return old;
> > >  }
> > >
> > > +static inline unsigned long pte_read(pte_t *p) { #ifdef
> > > +PTE_ATOMIC_UPDATES
> > > +       pte_t pte;
> > > +       pte_t tmp;
> > > +       __asm__ __volatile__ (
> > > +       "1:     ldarx   %0,0,%3\n"
> > > +       "       andi.   %1,%0,%4\n"
> > > +       "       bne-    1b\n"
> > > +       "       ori     %1,%0,%4\n"
> > > +       "       stdcx.  %1,0,%3\n"
> > > +       "       bne-    1b"
> > > +       : "=&r" (pte), "=&r" (tmp), "=m" (*p)
> > > +       : "r" (p), "i" (_PAGE_BUSY)
> > > +       : "cc");
> > > +
> > > +       return pte;
> > > +#else
> > > +       return pte_val(*p);
> > > +#endif
> > > +#endif
> > > +}
> > >  static inline int __ptep_test_and_clear_young(struct mm_struct *mm,
> > >                                               unsigned long addr,
> > > pte_t *ptep)
> >
> > Please leave a blank line between functions.
> >
> > >  {
> > > diff --git a/arch/powerpc/include/asm/pgtable.h
> > > b/arch/powerpc/include/asm/pgtable.h
> > > index 690c8c2..dad712c 100644
> > > --- a/arch/powerpc/include/asm/pgtable.h
> > > +++ b/arch/powerpc/include/asm/pgtable.h
> > > @@ -254,6 +254,45 @@ static inline pte_t
> > > *find_linux_pte_or_hugepte(pgd_t *pgdir, unsigned long ea,  }
> > > #endif
> > > /* !CONFIG_HUGETLB_PAGE */
> > >
> > > +static inline pte_t lookup_linux_pte(pgd_t *pgdir, unsigned long hva,
> > > +                                    int writing, unsigned long
> > > +*pte_sizep)
> >
> > The name implies that it just reads the PTE.  Setting accessed/dirty
> > shouldn't be an undocumented side-effect.
> 
> Ok, will rename and document.
> 
> > Why can't the caller do that (or a different function that the caller
> > calls afterward if desired)?
> 
> The current implementation in book3s is;
>  1) find a pte/hugepte
>  2) return null if pte not present
>  3) take _PAGE_BUSY lock
>  4) set accessed/dirty
>  5) clear _PAGE_BUSY.
> 
> What I tried was
> 1) find a pte/hugepte
> 2) return null if pte not present
> 3) return pte (not take lock by not setting _PAGE_BUSY)
> 
> 4) then user calls  __ptep_set_access_flags() to atomic update the
> dirty/accessed flags in pte.
> 
> - but the benchmark results were not good
> - Also can there be race as we do not take lock in step 3 and update in step 4 ?
> 
> >
> > Though even then you have the undocumented side effect of locking the
> > PTE on certain targets.
> >
> > > +{
> > > +       pte_t *ptep;
> > > +       pte_t pte;
> > > +       unsigned long ps = *pte_sizep;
> > > +       unsigned int shift;
> > > +
> > > +       ptep = find_linux_pte_or_hugepte(pgdir, hva, &shift);
> > > +       if (!ptep)
> > > +               return __pte(0);
> > > +       if (shift)
> > > +               *pte_sizep = 1ul << shift;
> > > +       else
> > > +               *pte_sizep = PAGE_SIZE;
> > > +
> > > +       if (ps > *pte_sizep)
> > > +               return __pte(0);
> > > +
> > > +       if (!pte_present(*ptep))
> > > +               return __pte(0);
> > > +
> > > +#ifdef CONFIG_PPC64
> > > +       /* Lock PTE (set _PAGE_BUSY) and read */
> > > +       pte = pte_read(ptep);
> > > +#else
> > > +       pte = pte_val(*ptep);
> > > +#endif
> >
> > What about 32-bit platforms that need atomic PTEs?
> 
> I called __ptep_set_access_flags() for both 32/64bit (for 64bit I was not
> calling pte_read()), which handles atomic updates. Somehow the benchmark result
> were not good, will try again.

Hi Pauls,

I am trying to me the Linux pte search and update generic so that this can be used for powerpc as well.

I am not sure which of the below two should be ok, please help

--------------------------- (1) This --------------------------
/*
 * Lock and read a linux PTE.  If it's present and writable, atomically
 * set dirty and referenced bits and return the PTE, otherwise return 0.
 */
static inline atomic_read_update_pte_flags(pte_t *ptep, int writing)
{
        pte_t pte;
#ifdef CONFIG_PPC64
        /* Lock PTE (set _PAGE_BUSY) and read
         * XXX: NOTE: Some Architectures (book3e) does not have pte Locking,
         * so not generic in that sense for all architecture
         */
        pte = pte_read(ptep);
#else
        /* XXX: do not see any read locking on 32 bit architecture */
        pte = pte_val(*ptep);
#endif
        if (pte_present(pte)) {
                pte = pte_mkyoung(pte);
                if (writing && pte_write(pte))
                        pte = pte_mkdirty(pte);
        }

        *ptep = __pte(pte); /* 64bit: Also unlock pte (clear _PAGE_BUSY) */

        return pte;
}


--------------------- (2) OR THIS ----------------------------------

/*
 * Read a linux PTE.  If it's present and writable, atomically
 * set dirty and referenced bits and return the PTE, otherwise return 0.
 */
static inline atomic_read_update_pte_flags(pte_t *ptep, int writing)
{
        pte_t pte;
        pte = pte_val(*ptep);
        if (pte_present(pte)) {
                pte = pte_mkyoung(pte);
                if (writing && pte_write(pte))
                        pte = pte_mkdirty(pte);
        }

        __ptep_set_access_flags(ptep, pte);

        return pte_val(*ptep);
}

Thanks
-Bharat

> 
> Thanks
> -Bharat
> >
> > -Scott
> >

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