Commit 002c5f73 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
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KVM: x86/mmu: Reintroduce fast invalidate/zap for flushing memslot 



James Harvey reported a livelock that was introduced by commit
d012a06a ("Revert "KVM: x86/mmu: Zap only the relevant pages when
removing a memslot"").

The livelock occurs because kvm_mmu_zap_all() as it exists today will
voluntarily reschedule and drop KVM's mmu_lock, which allows other vCPUs
to add shadow pages.  With enough vCPUs, kvm_mmu_zap_all() can get stuck
in an infinite loop as it can never zap all pages before observing lock
contention or the need to reschedule.  The equivalent of kvm_mmu_zap_all()
that was in use at the time of the reverted commit (4e103134, "KVM:
x86/mmu: Zap only the relevant pages when removing a memslot") employed
a fast invalidate mechanism and was not susceptible to the above livelock.

There are three ways to fix the livelock:

- Reverting the revert (commit d012a06a) is not a viable option as
  the revert is needed to fix a regression that occurs when the guest has
  one or more assigned devices.  It's unlikely we'll root cause the device
  assignment regression soon enough to fix the regression timely.

- Remove the conditional reschedule from kvm_mmu_zap_all().  However, although
  removing the reschedule would be a smaller code change, it's less safe
  in the sense that the resulting kvm_mmu_zap_all() hasn't been used in
  the wild for flushing memslots since the fast invalidate mechanism was
  introduced by commit 6ca18b69 ("KVM: x86: use the fast way to
  invalidate all pages"), back in 2013.

- Reintroduce the fast invalidate mechanism and use it when zapping shadow
  pages in response to a memslot being deleted/moved, which is what this
  patch does.

For all intents and purposes, this is a revert of commit ea145aac
("Revert "KVM: MMU: fast invalidate all pages"") and a partial revert of
commit 7390de1e ("Revert "KVM: x86: use the fast way to invalidate
all pages""), i.e. restores the behavior of commit 5304b8d3 ("KVM:
MMU: fast invalidate all pages") and commit 6ca18b69 ("KVM: x86:
use the fast way to invalidate all pages") respectively.

Fixes: d012a06a ("Revert "KVM: x86/mmu: Zap only the relevant pages when removing a memslot"")
Reported-by: default avatarJames Harvey <jamespharvey20@gmail.com>
Cc: Alex Willamson <alex.williamson@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 541ab2ae

arch/x86/include/asm/kvm_host.h

+2 −0
Original line number Diff line number Diff line
@@ -335,6 +335,7 @@ struct kvm_mmu_page { 
	int root_count;          /* Currently serving as active root */

	unsigned int unsync_children;

	struct kvm_rmap_head parent_ptes; /* rmap pointers to parent sptes */

	unsigned long mmu_valid_gen;

	DECLARE_BITMAP(unsync_child_bitmap, 512);



#ifdef CONFIG_X86_32
@@ -856,6 +857,7 @@ struct kvm_arch { 
	unsigned long n_requested_mmu_pages;

	unsigned long n_max_mmu_pages;

	unsigned int indirect_shadow_pages;

	unsigned long mmu_valid_gen;

	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];

	/*

	 * Hash table of struct kvm_mmu_page.

arch/x86/kvm/mmu.c

+99 −2
Original line number Diff line number Diff line
@@ -2095,6 +2095,12 @@ static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu, int direct 
	if (!direct)

		sp->gfns = mmu_memory_cache_alloc(&vcpu->arch.mmu_page_cache);

	set_page_private(virt_to_page(sp->spt), (unsigned long)sp);



	/*

	 * active_mmu_pages must be a FIFO list, as kvm_zap_obsolete_pages()

	 * depends on valid pages being added to the head of the list.  See

	 * comments in kvm_zap_obsolete_pages().

	 */

	list_add(&sp->link, &vcpu->kvm->arch.active_mmu_pages);

	kvm_mod_used_mmu_pages(vcpu->kvm, +1);

	return sp;
@@ -2244,7 +2250,7 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm, 
#define for_each_valid_sp(_kvm, _sp, _gfn)				\

	hlist_for_each_entry(_sp,					\

	  &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)], hash_link) \

		if ((_sp)->role.invalid) {    \

		if (is_obsolete_sp((_kvm), (_sp)) || (_sp)->role.invalid) {    \

		} else



#define for_each_gfn_indirect_valid_sp(_kvm, _sp, _gfn)			\
@@ -2301,6 +2307,11 @@ static void kvm_mmu_audit(struct kvm_vcpu *vcpu, int point) { } 
static void mmu_audit_disable(void) { }

#endif



static bool is_obsolete_sp(struct kvm *kvm, struct kvm_mmu_page *sp)

{

	return unlikely(sp->mmu_valid_gen != kvm->arch.mmu_valid_gen);

}



static bool kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,

			 struct list_head *invalid_list)

{
@@ -2525,6 +2536,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, 
		if (level > PT_PAGE_TABLE_LEVEL && need_sync)

			flush |= kvm_sync_pages(vcpu, gfn, &invalid_list);

	}

	sp->mmu_valid_gen = vcpu->kvm->arch.mmu_valid_gen;

	clear_page(sp->spt);

	trace_kvm_mmu_get_page(sp, true);
@@ -4233,6 +4245,13 @@ static bool fast_cr3_switch(struct kvm_vcpu *vcpu, gpa_t new_cr3, 
			return false;



		if (cached_root_available(vcpu, new_cr3, new_role)) {

			/*

			 * It is possible that the cached previous root page is

			 * obsolete because of a change in the MMU generation

			 * number. However, changing the generation number is

			 * accompanied by KVM_REQ_MMU_RELOAD, which will free

			 * the root set here and allocate a new one.

			 */

			kvm_make_request(KVM_REQ_LOAD_CR3, vcpu);

			if (!skip_tlb_flush) {

				kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
@@ -5649,11 +5668,89 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu) 
	return alloc_mmu_pages(vcpu);

}





static void kvm_zap_obsolete_pages(struct kvm *kvm)

{

	struct kvm_mmu_page *sp, *node;

	LIST_HEAD(invalid_list);

	int ign;



restart:

	list_for_each_entry_safe_reverse(sp, node,

	      &kvm->arch.active_mmu_pages, link) {

		/*

		 * No obsolete valid page exists before a newly created page

		 * since active_mmu_pages is a FIFO list.

		 */

		if (!is_obsolete_sp(kvm, sp))

			break;



		/*

		 * Do not repeatedly zap a root page to avoid unnecessary

		 * KVM_REQ_MMU_RELOAD, otherwise we may not be able to

		 * progress:

		 *    vcpu 0                        vcpu 1

		 *                         call vcpu_enter_guest():

		 *                            1): handle KVM_REQ_MMU_RELOAD

		 *                                and require mmu-lock to

		 *                                load mmu

		 * repeat:

		 *    1): zap root page and

		 *        send KVM_REQ_MMU_RELOAD

		 *

		 *    2): if (cond_resched_lock(mmu-lock))

		 *

		 *                            2): hold mmu-lock and load mmu

		 *

		 *                            3): see KVM_REQ_MMU_RELOAD bit

		 *                                on vcpu->requests is set

		 *                                then return 1 to call

		 *                                vcpu_enter_guest() again.

		 *            goto repeat;

		 *

		 * Since we are reversely walking the list and the invalid

		 * list will be moved to the head, skip the invalid page

		 * can help us to avoid the infinity list walking.

		 */

		if (sp->role.invalid)

			continue;



		if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {

			kvm_mmu_commit_zap_page(kvm, &invalid_list);

			cond_resched_lock(&kvm->mmu_lock);

			goto restart;

		}



		if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list, &ign))

			goto restart;

	}



	kvm_mmu_commit_zap_page(kvm, &invalid_list);

}



/*

 * Fast invalidate all shadow pages and use lock-break technique

 * to zap obsolete pages.

 *

 * It's required when memslot is being deleted or VM is being

 * destroyed, in these cases, we should ensure that KVM MMU does

 * not use any resource of the being-deleted slot or all slots

 * after calling the function.

 */

static void kvm_mmu_zap_all_fast(struct kvm *kvm)

{

	spin_lock(&kvm->mmu_lock);

	kvm->arch.mmu_valid_gen++;



	kvm_zap_obsolete_pages(kvm);

	spin_unlock(&kvm->mmu_lock);

}



static void kvm_mmu_invalidate_zap_pages_in_memslot(struct kvm *kvm,

			struct kvm_memory_slot *slot,

			struct kvm_page_track_notifier_node *node)

{

	kvm_mmu_zap_all(kvm);

	kvm_mmu_zap_all_fast(kvm);

}



void kvm_mmu_init_vm(struct kvm *kvm)

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