linux-kernel - Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled

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Message-ID: <333451bd-4730-4ebb-f76c-28fa5d0e1f7d@redhat.com>
Date:   Mon, 26 Oct 2020 09:23:31 +1100
From:   Gavin Shan <gshan@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        will@...nel.org, alexandru.elisei@....com
Subject: Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled

Hi Marc,

On 10/25/20 8:52 PM, Marc Zyngier wrote:
> On Sun, 25 Oct 2020 01:27:37 +0100,
> Gavin Shan <gshan@...hat.com> wrote:
>>
>> The 52-bits physical address is disabled until CONFIG_ARM64_PA_BITS_52
>> is chosen. This uses option for that check, to avoid the unconditional
>> check on PAGE_SHIFT in the hot path and thus save some CPU cycles.
> 
> PAGE_SHIFT is known at compile time, and this code is dropped by the
> compiler if the selected page size is not 64K. This patch really only
> makes the code slightly less readable and the "CPU cycles" argument
> doesn't hold at all.
> 
> So what are you trying to solve exactly?
> 

There are two points covered by the patch: (1) The 52-bits physical address
is visible only when CONFIG_ARM64_PA_BITS_52 is enabled in arch/arm64 code.
The code looks consistent with this option used here. (2) I had the assumption
that gcc doesn't optimize the code and PAGE_SHIFT is always checked in order
to get higher 4 physical address bits, but you said gcc should optimize the
code accordingly. However, it would be still nice to make the code explicit.

Thanks,
Gavin

>>
>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>> ---
>>   arch/arm64/kvm/hyp/pgtable.c | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
>> index 0cdf6e461cbd..fd850353ee89 100644
>> --- a/arch/arm64/kvm/hyp/pgtable.c
>> +++ b/arch/arm64/kvm/hyp/pgtable.c
>> @@ -132,8 +132,9 @@ static u64 kvm_pte_to_phys(kvm_pte_t pte)
>>   {
>>   	u64 pa = pte & KVM_PTE_ADDR_MASK;
>>   
>> -	if (PAGE_SHIFT == 16)
>> -		pa |= FIELD_GET(KVM_PTE_ADDR_51_48, pte) << 48;
>> +#ifdef CONFIG_ARM64_PA_BITS_52
>> +	pa |= FIELD_GET(KVM_PTE_ADDR_51_48, pte) << 48;
>> +#endif
>>   
>>   	return pa;
>>   }
>> @@ -142,8 +143,9 @@ static kvm_pte_t kvm_phys_to_pte(u64 pa)
>>   {
>>   	kvm_pte_t pte = pa & KVM_PTE_ADDR_MASK;
>>   
>> -	if (PAGE_SHIFT == 16)
>> -		pte |= FIELD_PREP(KVM_PTE_ADDR_51_48, pa >> 48);
>> +#ifdef CONFIG_ARM64_PA_BITS_52
>> +	pte |= FIELD_PREP(KVM_PTE_ADDR_51_48, pa >> 48);
>> +#endif
>>   
>>   	return pte;
>>   }
>> -- 
>> 2.23.0
>>
>>
>