Commit ba6a3541 authored by Paolo Bonzini's avatar Paolo Bonzini Committed by Gleb Natapov
Browse files

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: default avatarXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: default avatarGleb Natapov <gleb@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 3261107e

arch/x86/kvm/paging_tmpl.h

+19 −1
Original line number Diff line number Diff line
@@ -99,6 +99,7 @@ struct guest_walker { 
	pt_element_t prefetch_ptes[PTE_PREFETCH_NUM];

	gpa_t pte_gpa[PT_MAX_FULL_LEVELS];

	pt_element_t __user *ptep_user[PT_MAX_FULL_LEVELS];

	bool pte_writable[PT_MAX_FULL_LEVELS];

	unsigned pt_access;

	unsigned pte_access;

	gfn_t gfn;
@@ -235,6 +236,22 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu, 
		if (pte == orig_pte)

			continue;



		/*

		 * If the slot is read-only, simply do not process the accessed

		 * and dirty bits.  This is the correct thing to do if the slot

		 * is ROM, and page tables in read-as-ROM/write-as-MMIO slots

		 * are only supported if the accessed and dirty bits are already

		 * set in the ROM (so that MMIO writes are never needed).

		 *

		 * Note that NPT does not allow this at all and faults, since

		 * it always wants nested page table entries for the guest

		 * page tables to be writable.  And EPT works but will simply

		 * overwrite the read-only memory to set the accessed and dirty

		 * bits.

		 */

		if (unlikely(!walker->pte_writable[level - 1]))

			continue;



		ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index, orig_pte, pte);

		if (ret)

			return ret;
@@ -309,7 +326,8 @@ retry_walk: 
			goto error;

		real_gfn = gpa_to_gfn(real_gfn);



		host_addr = gfn_to_hva(vcpu->kvm, real_gfn);

		host_addr = gfn_to_hva_prot(vcpu->kvm, real_gfn,

					    &walker->pte_writable[walker->level - 1]);

		if (unlikely(kvm_is_error_hva(host_addr)))

			goto error;

include/linux/kvm_host.h

+1 −0
Original line number Diff line number Diff line
@@ -533,6 +533,7 @@ int gfn_to_page_many_atomic(struct kvm *kvm, gfn_t gfn, struct page **pages, 


struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn);

unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn);

unsigned long gfn_to_hva_prot(struct kvm *kvm, gfn_t gfn, bool *writable);

unsigned long gfn_to_hva_memslot(struct kvm_memory_slot *slot, gfn_t gfn);

void kvm_release_page_clean(struct page *page);

void kvm_release_page_dirty(struct page *page);

virt/kvm/kvm_main.c

+9 −5
Original line number Diff line number Diff line
@@ -1058,11 +1058,15 @@ unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn) 
EXPORT_SYMBOL_GPL(gfn_to_hva);



/*

 * The hva returned by this function is only allowed to be read.

 * It should pair with kvm_read_hva() or kvm_read_hva_atomic().

 * If writable is set to false, the hva returned by this function is only

 * allowed to be read.

 */

static unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn)

unsigned long gfn_to_hva_prot(struct kvm *kvm, gfn_t gfn, bool *writable)

{

	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);

	if (writable)

		*writable = !memslot_is_readonly(slot);



	return __gfn_to_hva_many(gfn_to_memslot(kvm, gfn), gfn, NULL, false);

}
@@ -1430,7 +1434,7 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset, 
	int r;

	unsigned long addr;



	addr = gfn_to_hva_read(kvm, gfn);

	addr = gfn_to_hva_prot(kvm, gfn, NULL);

	if (kvm_is_error_hva(addr))

		return -EFAULT;

	r = kvm_read_hva(data, (void __user *)addr + offset, len);
@@ -1468,7 +1472,7 @@ int kvm_read_guest_atomic(struct kvm *kvm, gpa_t gpa, void *data, 
	gfn_t gfn = gpa >> PAGE_SHIFT;

	int offset = offset_in_page(gpa);



	addr = gfn_to_hva_read(kvm, gfn);

	addr = gfn_to_hva_prot(kvm, gfn, NULL);

	if (kvm_is_error_hva(addr))

		return -EFAULT;

	pagefault_disable();

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