Messages in this thread | | | Date | Wed, 13 Jan 2021 15:44:15 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v2 2/3] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler |
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On Wed, Dec 16, 2020 at 08:28:43PM +0800, Yanan Wang wrote: > In dirty-logging, or dirty-logging-stopped time, even normal running > time of a guest configed with huge mappings and numbers of vCPUs, > translation faults by different vCPUs on the same GPA could occur > successively almost at the same time. There are two reasons for it. > > (1) If there are some vCPUs accessing the same GPA at the same time and > the leaf PTE is not set yet, then they will all cause translation faults > and the first vCPU holding mmu_lock will set valid leaf PTE, and the > others will later update the old PTE with a new one if they are different. > > (2) When changing a leaf entry or a table entry with break-before-make, > if there are some vCPUs accessing the same GPA just catch the moment when > the target PTE is set invalid in a BBM procedure coincidentally, they will > all cause translation faults and will later update the old PTE with a new > one if they are different. > > The worst case can be like this: vCPU A causes a translation fault with RW > prot and sets the leaf PTE with RW permissions, and then the next vCPU B > with RO prot updates the PTE back to RO permissions with break-before-make. > And the BBM-invalid moment may trigger more unnecessary translation faults, > then some useless small loops might occur which could lead to vCPU stuck. > > To avoid unnecessary update and small loops, add prejudgement in the > translation fault handler: 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. > > Signed-off-by: Yanan Wang <wangyanan55@huawei.com> > --- > arch/arm64/kvm/hyp/pgtable.c | 28 +++++++++++++++++++--------- > 1 file changed, 19 insertions(+), 9 deletions(-) > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index 350f9f810930..8225ced49bad 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; > @@ -460,7 +464,7 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot, > return 0; > } > > -static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level, > +static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level, > kvm_pte_t *ptep, > struct stage2_map_data *data) > { > @@ -469,13 +473,18 @@ static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level, > struct page *page = virt_to_page(ptep); > > if (!kvm_block_mapping_supported(addr, end, phys, level)) > - return false; > + return 1;
It would probably be cleaner to return another error code here, as we have failed to install a mapping (e.g. E2BIG or perhaps more perversely, ENOTBLK). Then the caller can decide to install a trable.
Will
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