Re: [RFC PATCH] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler
From: Marc Zyngier <maz@kernel.org>
Date: 2020-12-15 13:19:19
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Hi Yanan, On 2020-12-14 07:20, wangyanan (Y) wrote:
quoted hunk ↗ jump to hunk
diff --git a/arch/arm64/kvm/hyp/pgtable.cb/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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