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

[Gavin Shan]

Hi Marc,

On 10/25/20 8:52 PM, Marc Zyngier wrote:
> On Sun, 25 Oct 2020 01:27:37 +0100,
> Gavin Shan <gshan@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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