[PATCH 5.2 46/85] KVM: x86/mmu: Reintroduce fast invalidate/zap for flushing memslot

Greg Kroah-Hartman
2019-09-18T06:28:24Z

From: Sean Christopherson <sean.j.christopherson@intel.com>

commit 002c5f73c508f7df5681bda339831c27f3c1aef4 upstream.

James Harvey reported a livelock that was introduced by commit
d012a06ab1d23 ("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 (4e103134b8623, "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 d012a06ab1d23) 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 6ca18b6950f8d ("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 ea145aacf4ae8
("Revert "KVM: MMU: fast invalidate all pages"") and a partial revert of
commit 7390de1e99a70 ("Revert "KVM: x86: use the fast way to invalidate
all pages""), i.e. restores the behavior of commit 5304b8d37c2a5 ("KVM:
MMU: fast invalidate all pages") and commit 6ca18b6950f8d ("KVM: x86:
use the fast way to invalidate all pages") respectively.

Fixes: d012a06ab1d23 ("Revert "KVM: x86/mmu: Zap only the relevant pages when removing a memslot"")
Reported-by: James 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: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/include/asm/kvm_host.h | 2
arch/x86/kvm/mmu.c | 101 +++++++++++++++++++++++++++++++++++++++-
2 files changed, 101 insertions(+), 2 deletions(-)

--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -333,6 +333,7 @@ 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
@@ -851,6 +852,7 @@ 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.
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2066,6 +2066,12 @@ static struct kvm_mmu_page *kvm_mmu_allo
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;
@@ -2215,7 +2221,7 @@ static void kvm_mmu_commit_zap_page(stru
#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) \
@@ -2272,6 +2278,11 @@ static void kvm_mmu_audit(struct kvm_vcp
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)
{
@@ -2496,6 +2507,7 @@ static struct kvm_mmu_page *kvm_mmu_get_
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);

@@ -4229,6 +4241,13 @@ static bool fast_cr3_switch(struct kvm_v
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);
@@ -5645,11 +5664,89 @@ 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)