Messages in this thread Patch in this message | | | Date | Fri, 11 Dec 2020 09:53:37 +0000 | From | Will Deacon <> | 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 Fri, Dec 11, 2020 at 04:01:15PM +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 choose to update the leaf PTE or not. > > (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 choose to update > the leaf PTE or not. > > The worst case can be like this: some vCPUs cause translation faults > on the same GPA with different prots, they will fight each other by > changing back access permissions of the PTE with break-before-make. > And the BBM-invalid moment might trigger more unnecessary translation > faults. As a result, some useless small loops will occur, which could > lead to vCPU stuck. > > To avoid unnecessary update and small loops, add prejudgement in the > translation fault handler: Skip updating the valid leaf PTE if we are > trying to recreate exactly the same mapping or to reduce access > permissions only(such as RW-->RO). And update the valid leaf PTE without > break-before-make if we are trying to add more permissions only. > > Signed-off-by: Yanan Wang <wangyanan55@huawei.com> > --- > arch/arm64/kvm/hyp/pgtable.c | 73 +++++++++++++++++++++++++----------- > 1 file changed, 52 insertions(+), 21 deletions(-)
Cheers for this. Given that this patch is solving a few different problems, do you think you could split it up please? That would certainly make it much easier to review, as there's quite a lot going on here. A chunk of the changes seem to be the diff I posted previously:
https://lore.kernel.org/r/20201201141632.GC26973@willie-the-truck
so maybe that could be its own patch?
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > index 23a01dfcb27a..f8b3248cef1c 100644 > --- a/arch/arm64/kvm/hyp/pgtable.c > +++ b/arch/arm64/kvm/hyp/pgtable.c > @@ -45,6 +45,8 @@ > > #define KVM_PTE_LEAF_ATTR_HI_S2_XN BIT(54) > > +#define KVM_PTE_LEAF_ATTR_PERMS (GENMASK(7, 6) | BIT(54))
You only use this on the S2 path, so how about:
#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
or something like that?
> struct kvm_pgtable_walk_data { > struct kvm_pgtable *pgt; > struct kvm_pgtable_walker *walker; > @@ -170,10 +172,9 @@ static void kvm_set_table_pte(kvm_pte_t *ptep, kvm_pte_t *childp) > smp_store_release(ptep, pte); > } > > -static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, kvm_pte_t attr, > - u32 level) > +static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level) > { > - kvm_pte_t old = *ptep, pte = kvm_phys_to_pte(pa); > + kvm_pte_t pte = kvm_phys_to_pte(pa); > u64 type = (level == KVM_PGTABLE_MAX_LEVELS - 1) ? KVM_PTE_TYPE_PAGE : > KVM_PTE_TYPE_BLOCK; > > @@ -181,12 +182,7 @@ static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, kvm_pte_t attr, > pte |= FIELD_PREP(KVM_PTE_TYPE, type); > pte |= KVM_PTE_VALID; > > - /* Tolerate KVM recreating the exact same mapping. */ > - if (kvm_pte_valid(old)) > - return old == pte; > - > - smp_store_release(ptep, pte); > - return true; > + return pte; > } > > static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data, u64 addr, > @@ -341,12 +337,17 @@ static int hyp_map_set_prot_attr(enum kvm_pgtable_prot prot, > static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level, > kvm_pte_t *ptep, struct hyp_map_data *data) > { > + kvm_pte_t new, old = *ptep; > u64 granule = kvm_granule_size(level), phys = data->phys; > > if (!kvm_block_mapping_supported(addr, end, phys, level)) > return false; > > - WARN_ON(!kvm_set_valid_leaf_pte(ptep, phys, data->attr, level)); > + /* Tolerate KVM recreating the exact same mapping. */ > + new = kvm_init_valid_leaf_pte(phys, data->attr, level); > + if (old != new && !WARN_ON(kvm_pte_valid(old))) > + smp_store_release(ptep, new); > + > data->phys += granule; > return true; > } > @@ -461,25 +462,56 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot, > return 0; > } > > +static bool stage2_set_valid_leaf_pte_pre(u64 addr, u32 level, > + kvm_pte_t *ptep, kvm_pte_t new, > + struct stage2_map_data *data) > +{ > + kvm_pte_t old = *ptep, old_attr, new_attr; > + > + if ((old ^ new) & (~KVM_PTE_LEAF_ATTR_PERMS)) > + return false; > + > + /* > + * Skip updating if we are trying to recreate exactly the same mapping > + * or to reduce the access permissions only. And update the valid leaf > + * PTE without break-before-make if we are trying to add more access > + * permissions only. > + */ > + old_attr = (old & KVM_PTE_LEAF_ATTR_PERMS) ^ KVM_PTE_LEAF_ATTR_HI_S2_XN; > + new_attr = (new & KVM_PTE_LEAF_ATTR_PERMS) ^ KVM_PTE_LEAF_ATTR_HI_S2_XN; > + if (new_attr <= old_attr) > + return true;
I think this is a significant change in behaviour for kvm_pgtable_stage2_map() and I worry that it could catch somebody out in the future. Please can you update the kerneldoc in kvm_pgtable.h with a note about this?
Will
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