Messages in this thread | | | Subject | Re: [PATCH v2 2/3] KVM: arm64: Add prejudgement for relaxing permissions only case in stage2 translation fault handler | From | "wangyanan (Y)" <> | Date | Thu, 14 Jan 2021 17:28:09 +0800 |
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On 2021/1/13 23:44, Will Deacon wrote: > 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. > Ok, I will make some adjustment for this and post the v3 soon after.
Thanks,
Yanan.
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