Re: [PATCH 2/3] KVM: arm64: Don't map PUD huge page if it's not available

[Gavin Shan <gshan@redhat.com>]

Hi Marc,

On 10/25/20 9:05 PM, Marc Zyngier wrote:
> On Sun, 25 Oct 2020 01:27:38 +0100,
> Gavin Shan <gshan@redhat.com> wrote:
>>
>> PUD huge page isn't available when CONFIG_ARM64_4K_PAGES is disabled.
>> In this case, we needn't try to map the memory through PUD huge pages
>> to save some CPU cycles in the hot path.
>>
>> This also corrects the code style issue, which was introduced by
>> commit <523b3999e5f6> ("KVM: arm64: Try PMD block mappings if PUD mappings
>> are not supported").
>>
>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>> ---
>>   arch/arm64/kvm/mmu.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index a816cb8e619b..0f51585adc04 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -787,9 +787,11 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   		vma_shift = PAGE_SHIFT;
>>   	}
>>   
>> +#ifdef CONFIG_ARM64_4K_PAGES
>>   	if (vma_shift == PUD_SHIFT &&
>>   	    !fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
>> -	       vma_shift = PMD_SHIFT;
>> +		vma_shift = PMD_SHIFT;
>> +#endif
>>   
>>   	if (vma_shift == PMD_SHIFT &&
>>   	    !fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) {
> 
> 
> I really don't buy the "CPU cycles" argument here either. Can you
> actually measure any difference here?
> 
> You have taken a fault, gone through a full guest exit, triaged it,
> and are about to into a page mapping operation which may result in a
> TLBI, and reenter the guest. It only happen a handful of times per
> page over the lifetime of the guest unless you start swapping. Hot
> path? I don't think so.
> 

Thanks for the explanation. Agreed and I will drop this in v2.

Thanks,
Gavin