linux-kernel - Re: [PATCH 04/24] kvm: x86/mmu: change TDP MMU yield function returns to match cond

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Date:   Tue, 26 Jan 2021 15:11:57 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 04/24] kvm: x86/mmu: change TDP MMU yield function returns
 to match cond_resched

On 20/01/21 19:38, Sean Christopherson wrote:
> On Tue, Jan 12, 2021, Ben Gardon wrote:
>> Currently the TDP MMU yield / cond_resched functions either return
>> nothing or return true if the TLBs were not flushed. These are confusing
>> semantics, especially when making control flow decisions in calling
>> functions.
>>
>> To clean things up, change both functions to have the same
>> return value semantics as cond_resched: true if the thread yielded,
>> false if it did not. If the function yielded in the _flush_ version,
>> then the TLBs will have been flushed.
>>
>> Reviewed-by: Peter Feiner <pfeiner@...gle.com>
>> Signed-off-by: Ben Gardon <bgardon@...gle.com>
>> ---
>>   arch/x86/kvm/mmu/tdp_mmu.c | 38 +++++++++++++++++++++++++++++---------
>>   1 file changed, 29 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
>> index 2ef8615f9dba..b2784514ca2d 100644
>> --- a/arch/x86/kvm/mmu/tdp_mmu.c
>> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
>> @@ -413,8 +413,15 @@ static inline void tdp_mmu_set_spte_no_dirty_log(struct kvm *kvm,
>>   			 _mmu->shadow_root_level, _start, _end)
>>   
>>   /*
>> - * Flush the TLB if the process should drop kvm->mmu_lock.
>> - * Return whether the caller still needs to flush the tlb.
>> + * Flush the TLB and yield if the MMU lock is contended or this thread needs to
>> + * return control to the scheduler.
>> + *
>> + * If this function yields, it will also reset the tdp_iter's walk over the
>> + * paging structure and the calling function should allow the iterator to
>> + * continue its traversal from the paging structure root.
>> + *
>> + * Return true if this function yielded, the TLBs were flushed, and the
>> + * iterator's traversal was reset. Return false if a yield was not needed.
>>    */
>>   static bool tdp_mmu_iter_flush_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
>>   {
>> @@ -422,18 +429,30 @@ static bool tdp_mmu_iter_flush_cond_resched(struct kvm *kvm, struct tdp_iter *it
>>   		kvm_flush_remote_tlbs(kvm);
>>   		cond_resched_lock(&kvm->mmu_lock);
>>   		tdp_iter_refresh_walk(iter);
>> -		return false;
>> -	} else {
>>   		return true;
>> -	}
>> +	} else
>> +		return false;
> 
> Kernel style is to have curly braces on all branches if any branch has 'em.  Or,
> omit the else since the taken branch always returns.  I think I prefer the latter?
> 
>>   }
>>   
>> -static void tdp_mmu_iter_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
>> +/*
>> + * Yield if the MMU lock is contended or this thread needs to return control
>> + * to the scheduler.
>> + *
>> + * If this function yields, it will also reset the tdp_iter's walk over the
>> + * paging structure and the calling function should allow the iterator to
>> + * continue its traversal from the paging structure root.
>> + *
>> + * Return true if this function yielded and the iterator's traversal was reset.
>> + * Return false if a yield was not needed.
>> + */
>> +static bool tdp_mmu_iter_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
>>   {
>>   	if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
>>   		cond_resched_lock(&kvm->mmu_lock);
>>   		tdp_iter_refresh_walk(iter);
>> -	}
>> +		return true;
>> +	} else
>> +		return false;
> 
> Same here.
> 
>>   }
>>   
>>   /*
>> @@ -470,7 +489,8 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>>   		tdp_mmu_set_spte(kvm, &iter, 0);
>>   
>>   		if (can_yield)
>> -			flush_needed = tdp_mmu_iter_flush_cond_resched(kvm, &iter);
>> +			flush_needed = !tdp_mmu_iter_flush_cond_resched(kvm,
>> +									&iter);
> 
> As with the existing code, I'd let this poke out.  Alternatively, this could be
> written as:
> 
> 		flush_needed = !can_yield ||
> 			       !tdp_mmu_iter_flush_cond_resched(kvm, &iter);
> 
>>   		else
>>   			flush_needed = true;
>>   	}
>> @@ -1072,7 +1092,7 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
>>   
>>   		tdp_mmu_set_spte(kvm, &iter, 0);
>>   
>> -		spte_set = tdp_mmu_iter_flush_cond_resched(kvm, &iter);
>> +		spte_set = !tdp_mmu_iter_flush_cond_resched(kvm, &iter);
>>   	}
>>   
>>   	if (spte_set)
>> -- 
>> 2.30.0.284.gd98b1dd5eaa7-goog
>>
> 

Tweaked and queued, thanks.

Paolo