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KVM: mmu: allow page tables to be in read-only slots
Page tables in a read-only memory slot will currently cause a triple fault because the page walker uses gfn_to_hva and it fails on such a slot. OVMF uses such a page table; however, real hardware seems to be fine with that as long as the accessed/dirty bits are set. Save whether the slot is readonly, and later check it when updating the accessed and dirty bits. Reviewed-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> Reviewed-by: Gleb Natapov <gleb@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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@ -99,6 +99,7 @@ struct guest_walker {
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pt_element_t prefetch_ptes[PTE_PREFETCH_NUM];
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gpa_t pte_gpa[PT_MAX_FULL_LEVELS];
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pt_element_t __user *ptep_user[PT_MAX_FULL_LEVELS];
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bool pte_writable[PT_MAX_FULL_LEVELS];
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unsigned pt_access;
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unsigned pte_access;
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gfn_t gfn;
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@ -235,6 +236,22 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu,
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if (pte == orig_pte)
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continue;
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/*
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* If the slot is read-only, simply do not process the accessed
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* and dirty bits. This is the correct thing to do if the slot
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* is ROM, and page tables in read-as-ROM/write-as-MMIO slots
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* are only supported if the accessed and dirty bits are already
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* set in the ROM (so that MMIO writes are never needed).
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*
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* Note that NPT does not allow this at all and faults, since
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* it always wants nested page table entries for the guest
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* page tables to be writable. And EPT works but will simply
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* overwrite the read-only memory to set the accessed and dirty
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* bits.
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*/
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if (unlikely(!walker->pte_writable[level - 1]))
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continue;
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ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index, orig_pte, pte);
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if (ret)
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return ret;
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@ -309,7 +326,8 @@ retry_walk:
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goto error;
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real_gfn = gpa_to_gfn(real_gfn);
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host_addr = gfn_to_hva(vcpu->kvm, real_gfn);
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host_addr = gfn_to_hva_prot(vcpu->kvm, real_gfn,
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&walker->pte_writable[walker->level - 1]);
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if (unlikely(kvm_is_error_hva(host_addr)))
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goto error;
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@ -533,6 +533,7 @@ int gfn_to_page_many_atomic(struct kvm *kvm, gfn_t gfn, struct page **pages,
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struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn);
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unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn);
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unsigned long gfn_to_hva_prot(struct kvm *kvm, gfn_t gfn, bool *writable);
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unsigned long gfn_to_hva_memslot(struct kvm_memory_slot *slot, gfn_t gfn);
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void kvm_release_page_clean(struct page *page);
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void kvm_release_page_dirty(struct page *page);
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@ -1058,11 +1058,15 @@ unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn)
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EXPORT_SYMBOL_GPL(gfn_to_hva);
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/*
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* The hva returned by this function is only allowed to be read.
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* It should pair with kvm_read_hva() or kvm_read_hva_atomic().
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* If writable is set to false, the hva returned by this function is only
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* allowed to be read.
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*/
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static unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn)
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unsigned long gfn_to_hva_prot(struct kvm *kvm, gfn_t gfn, bool *writable)
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{
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struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
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if (writable)
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*writable = !memslot_is_readonly(slot);
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return __gfn_to_hva_many(gfn_to_memslot(kvm, gfn), gfn, NULL, false);
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}
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@ -1430,7 +1434,7 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
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int r;
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unsigned long addr;
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addr = gfn_to_hva_read(kvm, gfn);
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addr = gfn_to_hva_prot(kvm, gfn, NULL);
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if (kvm_is_error_hva(addr))
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return -EFAULT;
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r = kvm_read_hva(data, (void __user *)addr + offset, len);
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@ -1468,7 +1472,7 @@ int kvm_read_guest_atomic(struct kvm *kvm, gpa_t gpa, void *data,
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gfn_t gfn = gpa >> PAGE_SHIFT;
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int offset = offset_in_page(gpa);
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addr = gfn_to_hva_read(kvm, gfn);
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addr = gfn_to_hva_prot(kvm, gfn, NULL);
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if (kvm_is_error_hva(addr))
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return -EFAULT;
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pagefault_disable();
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