Messages in this thread | | | Subject | Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled | From | Gavin Shan <> | Date | Mon, 26 Oct 2020 09:23:31 +1100 |
| |
Hi Marc,
On 10/25/20 8:52 PM, Marc Zyngier wrote: > On Sun, 25 Oct 2020 01:27:37 +0100, > Gavin Shan <gshan@redhat.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@redhat.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 >> >> >
| |