Commit 9a967700 authored by Lai Jiangshan's avatar Lai Jiangshan Committed by Paolo Bonzini
Browse files

KVM: x86/mmu: Remove FNAME(is_self_change_mapping) 



Drop FNAME(is_self_change_mapping) and instead rely on
kvm_mmu_hugepage_adjust() to adjust the hugepage accordingly.  Prior to
commit 4cd071d1 ("KVM: x86/mmu: Move calls to thp_adjust() down a
level"), the hugepage adjustment was done before allocating new shadow
pages, i.e. failed to restrict the hugepage sizes if a new shadow page
resulted in account_shadowed() changing the disallowed hugepage tracking.

Removing FNAME(is_self_change_mapping) fixes a bug reported by Huang Hang
where KVM unnecessarily forces a 4KiB page.  FNAME(is_self_change_mapping)
has a defect in that it blindly disables _all_ hugepage mappings rather
than trying to reduce the size of the hugepage.  If the guest is writing
to a 1GiB page and the 1GiB is self-referential but a 2MiB page is not,
then KVM can and should create a 2MiB mapping.

Add a comment above the call to kvm_mmu_hugepage_adjust() to call out the
new dependency on adjusting the hugepage size after walking indirect PTEs.

Reported-by: default avatarHuang Hang <hhuang@linux.alibaba.com>
Signed-off-by: default avatarLai Jiangshan <jiangshan.ljs@antgroup.com>
Link: https://lore.kernel.org/r/20221213125538.81209-1-jiangshanlai@gmail.com


[sean: rework changelog after separating out the emulator change]
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20230202182817.407394-4-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 39fda5d8

arch/x86/kvm/mmu/paging_tmpl.h

+7 −44
Original line number Diff line number Diff line
@@ -690,6 +690,12 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, 
			fault->write_fault_to_shadow_pgtable = true;

	}



	/*

	 * Adjust the hugepage size _after_ resolving indirect shadow pages.

	 * KVM doesn't support mapping hugepages into the guest for gfns that

	 * are being shadowed by KVM, i.e. allocating a new shadow page may

	 * affect the allowed hugepage size.

	 */

	kvm_mmu_hugepage_adjust(vcpu, fault);



	trace_kvm_mmu_spte_requested(fault);
@@ -734,41 +740,6 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, 
	return RET_PF_RETRY;

}



 /*

 * To see whether the mapped gfn can write its page table in the current

 * mapping.

 *

 * It is the helper function of FNAME(page_fault). When guest uses large page

 * size to map the writable gfn which is used as current page table, we should

 * force kvm to use small page size to map it because new shadow page will be

 * created when kvm establishes shadow page table that stop kvm using large

 * page size. Do it early can avoid unnecessary #PF and emulation.

 *

 * Note: the PDPT page table is not checked for PAE-32 bit guest. It is ok

 * since the PDPT is always shadowed, that means, we can not use large page

 * size to map the gfn which is used as PDPT.

 */

static bool

FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,

			      struct guest_walker *walker, bool user_fault)

{

	int level;

	gfn_t mask = ~(KVM_PAGES_PER_HPAGE(walker->level) - 1);

	bool self_changed = false;



	if (!(walker->pte_access & ACC_WRITE_MASK ||

	    (!is_cr0_wp(vcpu->arch.mmu) && !user_fault)))

		return false;



	for (level = walker->level; level <= walker->max_level; level++) {

		gfn_t gfn = walker->gfn ^ walker->table_gfn[level - 1];



		self_changed |= !(gfn & mask);

	}



	return self_changed;

}



/*

 * Page fault handler.  There are several causes for a page fault:

 *   - there is no shadow pte for the guest pte
@@ -787,7 +758,6 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault 
{

	struct guest_walker walker;

	int r;

	bool is_self_change_mapping;



	pgprintk("%s: addr %lx err %x\n", __func__, fault->addr, fault->error_code);

	WARN_ON_ONCE(fault->is_tdp);
@@ -812,6 +782,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault 
	}



	fault->gfn = walker.gfn;

	fault->max_level = walker.level;

	fault->slot = kvm_vcpu_gfn_to_memslot(vcpu, fault->gfn);



	if (page_fault_handle_page_track(vcpu, fault)) {
@@ -823,14 +794,6 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault 
	if (r)

		return r;



	is_self_change_mapping = FNAME(is_self_change_mapping)(vcpu,

	      &walker, fault->user);



	if (is_self_change_mapping)

		fault->max_level = PG_LEVEL_4K;

	else

		fault->max_level = walker.level;



	r = kvm_faultin_pfn(vcpu, fault, walker.pte_access);

	if (r != RET_PF_CONTINUE)

		return r;