Messages in this thread | | | Date | Tue, 15 Dec 2020 13:18:35 +0000 | From | Marc Zyngier <> | Subject | Re: [RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler |
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Hi Yanan,
On 2020-12-14 07:20, wangyanan (Y) wrote:
> diff --git a/arch/arm64/kvm/hyp/pgtable.c > b/arch/arm64/kvm/hyp/pgtable.c > index a74a62283012..e3c6133567c4 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -45,6 +45,10 @@ > > #define KVM_PTE_LEAF_ATTR_HI_S2_XN BIT(54) > > +#define KVM_PTE_LEAF_ATTR_S2_PERMS (KVM_PTE_LEAF_ATTR_LO_S2_S2AP_R | \ > + KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W | \ > + KVM_PTE_LEAF_ATTR_HI_S2_XN) > + > struct kvm_pgtable_walk_data { > struct kvm_pgtable *pgt; > struct kvm_pgtable_walker *walker; > @@ -473,8 +477,13 @@ static bool stage2_map_walker_try_leaf(u64 addr, > u64 end, u32 level, > > new = kvm_init_valid_leaf_pte(phys, data->attr, level); > if (kvm_pte_valid(old)) { > - /* Tolerate KVM recreating the exact same mapping. */ > - if (old == new) > + /* > + * Skip updating the PTE with break-before-make if we > are trying > + * to recreate the exact same mapping or only change > the access > + * permissions. Actually, change of permissions will be > handled > + * through the relax_perms path next time if necessary. > + */ > + if (!((old ^ new) & (~KVM_PTE_LEAF_ATTR_S2_PERMS))) > goto out; > > /* There's an existing different valid leaf entry, so > perform
I think there is a bit more work to do on this.
One obvious issue is that we currently flag a page as dirty before handling the fault. With an early exit, we end-up having spurious dirty pages.
It's not a big deal, but I'd rather mark the page dirty after the mapping or the permission update having been successful (at the moment, it cannot fails).
Thanks,
M. -- Jazz is not dead. It just smells funny...
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