Messages in this thread | | | Subject | Re: [PATCH v5 6/6] KVM: arm64: Distinguish cases of memcache allocations completely | From | "wangyanan (Y)" <> | Date | Thu, 3 Jun 2021 20:52:36 +0800 |
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Hi Quentin,
On 2021/6/2 19:07, Quentin Perret wrote: > On Thursday 15 Apr 2021 at 19:50:32 (+0800), Yanan Wang wrote: >> With a guest translation fault, the memcache pages are not needed if KVM >> is only about to install a new leaf entry into the existing page table. >> And with a guest permission fault, the memcache pages are also not needed >> for a write_fault in dirty-logging time if KVM is only about to update >> the existing leaf entry instead of collapsing a block entry into a table. >> >> By comparing fault_granule and vma_pagesize, cases that require allocations >> from memcache and cases that don't can be distinguished completely. >> >> Signed-off-by: Yanan Wang <wangyanan55@huawei.com> >> --- >> arch/arm64/kvm/mmu.c | 25 ++++++++++++------------- >> 1 file changed, 12 insertions(+), 13 deletions(-) >> >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c >> index aa536392b308..9e35aa5d29f2 100644 >> --- a/arch/arm64/kvm/mmu.c >> +++ b/arch/arm64/kvm/mmu.c >> @@ -895,19 +895,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, >> gfn = fault_ipa >> PAGE_SHIFT; >> mmap_read_unlock(current->mm); >> >> - /* >> - * Permission faults just need to update the existing leaf entry, >> - * and so normally don't require allocations from the memcache. The >> - * only exception to this is when dirty logging is enabled at runtime >> - * and a write fault needs to collapse a block entry into a table. >> - */ >> - if (fault_status != FSC_PERM || (logging_active && write_fault)) { >> - ret = kvm_mmu_topup_memory_cache(memcache, >> - kvm_mmu_cache_min_pages(kvm)); >> - if (ret) >> - return ret; >> - } >> - >> mmu_seq = vcpu->kvm->mmu_notifier_seq; >> /* >> * Ensure the read of mmu_notifier_seq happens before we call >> @@ -970,6 +957,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, >> else if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC)) >> prot |= KVM_PGTABLE_PROT_X; >> >> + /* >> + * Allocations from the memcache are required only when granule of the >> + * lookup level where the guest fault happened exceeds vma_pagesize, >> + * which means new page tables will be created in the fault handlers. >> + */ >> + if (fault_granule > vma_pagesize) { >> + ret = kvm_mmu_topup_memory_cache(memcache, >> + kvm_mmu_cache_min_pages(kvm)); >> + if (ret) >> + return ret; >> + } > You're now doing the top-up in the kvm->mmu_lock critical section. Isn't > this more or less what we try to avoid by using a memory cache? Oh, right!
This patch intended to clean up the code and avoid the unnecessary top-ups, but it's a bad idea to do the top-up when holding mmu_lock. I will rearrange this part and keep it where it should be.
Thanks, Yanan > Thanks, > Quentin > .
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