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

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 19 Jun 2013 18:00:43 +0530

Michael Neuling <[email-masked]> writes:

> Aneesh Kumar K.V <[email-masked]> wrote:
>
>> From: "Aneesh Kumar K.V" <[email-masked]>
>> 
>> 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 <[email-masked]>
>> ---
>>  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?  

In a way yes. Instead of lock and read as it was before, it is now done
via cmpxchg which still use ldarx/stdcx

>
>>   */
>> -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.

No that would be wrong. (I did that in an earlier version :).We really
don't want the below cmpxchg to run if we find _PAGE_BUSY.

>   
>> +
>> +#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?
>

it is actually the shift bits represending hugepage size. We set it to 0
if we don't find hugepage in find_linux_pte_or_hugepte. May be something
like hugepage_shift is better ?

-aneesh