Re: [PATCH] powerpc/kvm: Handle transparent hugepage in KVM

[Michael Neuling <mikey@neuling.org>]

Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> wrote:

> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> 
> We can find pte that are splitting while walking page tables. Return
> None pte in that case.

Can you expand on this more please.  There are a lot of details below
like removing a ldarx/stdcx loop that should be better described here.

> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/kvm_book3s_64.h | 51 ++++++++++++++++++--------------
>  arch/powerpc/kvm/book3s_64_mmu_hv.c      |  7 +++--
>  arch/powerpc/kvm/book3s_hv_rm_mmu.c      |  4 +--
>  3 files changed, 34 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_book3s_64.h b/arch/powerpc/include/asm/kvm_book3s_64.h
> index 9c1ff33..ce20f7e 100644
> --- a/arch/powerpc/include/asm/kvm_book3s_64.h
> +++ b/arch/powerpc/include/asm/kvm_book3s_64.h
> @@ -162,33 +162,40 @@ static inline int hpte_cache_flags_ok(unsigned long ptel, unsigned long io_type)
>   * 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.

This is comment still valid now the ldarx/stdcx is gone?  

>   */
> -static inline pte_t kvmppc_read_update_linux_pte(pte_t *p, int writing)
> +static inline pte_t kvmppc_read_update_linux_pte(pte_t *ptep, int writing,
> +						 unsigned int hugepage)
>  {
> -	pte_t pte, tmp;
> -
> -	/* wait until _PAGE_BUSY is clear then set it atomically */
> -	__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");
> -
> -	if (pte_present(pte)) {
> -		pte = pte_mkyoung(pte);
> -		if (writing && pte_write(pte))
> -			pte = pte_mkdirty(pte);
> -	}
> +	pte_t old_pte, new_pte = __pte(0);
> +repeat:
> +	do {
> +		old_pte = pte_val(*ptep);
> +		/*
> +		 * wait until _PAGE_BUSY is clear then set it atomically
> +		 */
> +		if (unlikely(old_pte & _PAGE_BUSY))
> +			goto repeat;

continue here?  Please don't create looping primitives.
  
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +		/* If hugepage and is trans splitting return None */
> +		if (unlikely(hugepage &&
> +			     pmd_trans_splitting(pte_pmd(old_pte))))

Comment looks much like the code... seems redundant.

> +			return __pte(0);
> +#endif
>  
> -	*p = pte;	/* clears _PAGE_BUSY */
> +		/* If pte is not present return None */
> +		if (unlikely(!(old_pte & _PAGE_PRESENT)))
> +			return __pte(0);
>  
> -	return pte;
> +		new_pte = pte_mkyoung(old_pte);
> +		if (writing && pte_write(old_pte))
> +			new_pte = pte_mkdirty(new_pte);
> +
> +	} while (old_pte != __cmpxchg_u64((unsigned long *)ptep,
> +					  old_pte, new_pte));
> +	return new_pte;
>  }
>  
> +

Whitespace

>  /* Return HPTE cache control bits corresponding to Linux pte bits */
>  static inline unsigned long hpte_cache_bits(unsigned long pte_val)
>  {
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 5880dfb..e1a9415 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -675,6 +675,7 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  		}
>  		/* if the guest wants write access, see if that is OK */
>  		if (!writing && hpte_is_writable(r)) {
> +			unsigned int shift;
>  			pte_t *ptep, pte;
>  
>  			/*
> @@ -683,9 +684,9 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  			 */
>  			rcu_read_lock_sched();
>  			ptep = find_linux_pte_or_hugepte(current->mm->pgd,
> -							 hva, NULL);
> -			if (ptep && pte_present(*ptep)) {
> -				pte = kvmppc_read_update_linux_pte(ptep, 1);
> +							 hva, &shift);
> +			if (ptep) {
> +				pte = kvmppc_read_update_linux_pte(ptep, 1, shift);
>  				if (pte_write(pte))
>  					write_ok = 1;
>  			}
> diff --git a/arch/powerpc/kvm/book3s_hv_rm_mmu.c b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> index dcf892d..39ae723 100644
> --- a/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> +++ b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> @@ -150,9 +150,7 @@ static pte_t lookup_linux_pte(pgd_t *pgdir, unsigned long hva,
>  		*pte_sizep = PAGE_SIZE;
>  	if (ps > *pte_sizep)
>  		return __pte(0);
> -	if (!pte_present(*ptep))
> -		return __pte(0);
> -	return kvmppc_read_update_linux_pte(ptep, writing);
> +	return kvmppc_read_update_linux_pte(ptep, writing, shift);

'shift' goes into the new 'hugepage' parameter?  Doesn't seem logical?
Can we harmonise the name to make it less confusing?

Mikey

>  }
>  
>  static inline void unlock_hpte(unsigned long *hpte, unsigned long hpte_v)
> -- 
> 1.8.1.2
> 
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