Re: [PATCH v5 6/6] KVM: arm64: Distinguish cases of memcache allocations completely

["wangyanan (Y)" <wangyanan55@huawei.com>]

Hi Quentin,

On 2021/6/2 19:07, Quentin Perret wrote:
> On Thursday 15 Apr 2021 at 19:50:32 (+0800), Yanan Wang wrote:
>> With a guest translation fault, the memcache pages are not needed if KVM
>> is only about to install a new leaf entry into the existing page table.
>> And with a guest permission fault, the memcache pages are also not needed
>> for a write_fault in dirty-logging time if KVM is only about to update
>> the existing leaf entry instead of collapsing a block entry into a table.
>>
>> By comparing fault_granule and vma_pagesize, cases that require allocations
>> from memcache and cases that don't can be distinguished completely.
>>
>> Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
>> ---
>>   arch/arm64/kvm/mmu.c | 25 ++++++++++++-------------
>>   1 file changed, 12 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index aa536392b308..9e35aa5d29f2 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -895,19 +895,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   	gfn = fault_ipa >> PAGE_SHIFT;
>>   	mmap_read_unlock(current->mm);
>>   
>> -	/*
>> -	 * Permission faults just need to update the existing leaf entry,
>> -	 * and so normally don't require allocations from the memcache. The
>> -	 * only exception to this is when dirty logging is enabled at runtime
>> -	 * and a write fault needs to collapse a block entry into a table.
>> -	 */
>> -	if (fault_status != FSC_PERM || (logging_active && write_fault)) {
>> -		ret = kvm_mmu_topup_memory_cache(memcache,
>> -						 kvm_mmu_cache_min_pages(kvm));
>> -		if (ret)
>> -			return ret;
>> -	}
>> -
>>   	mmu_seq = vcpu->kvm->mmu_notifier_seq;
>>   	/*
>>   	 * Ensure the read of mmu_notifier_seq happens before we call
>> @@ -970,6 +957,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   	else if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC))
>>   		prot |= KVM_PGTABLE_PROT_X;
>>   
>> +	/*
>> +	 * Allocations from the memcache are required only when granule of the
>> +	 * lookup level where the guest fault happened exceeds vma_pagesize,
>> +	 * which means new page tables will be created in the fault handlers.
>> +	 */
>> +	if (fault_granule > vma_pagesize) {
>> +		ret = kvm_mmu_topup_memory_cache(memcache,
>> +						 kvm_mmu_cache_min_pages(kvm));
>> +		if (ret)
>> +			return ret;
>> +	}
> You're now doing the top-up in the kvm->mmu_lock critical section. Isn't
> this more or less what we try to avoid by using a memory cache?
Oh, right!

This patch intended to clean up the code and avoid the unnecessary top-ups,
but it's a bad idea to do the top-up when holding mmu_lock. I will rearrange
this part and keep it where it should be.

Thanks,
Yanan
> Thanks,
> Quentin
> .