From: David Woodhouse dwmw2@infradead.org
commit 7e2175ebd695f17860c5bd4ad7616cce12ed4591 upstream.
In commit b043138246a4 ("x86/KVM: Make sure KVM_VCPU_FLUSH_TLB flag is not missed") we switched to using a gfn_to_pfn_cache for accessing the guest steal time structure in order to allow for an atomic xchg of the preempted field. This has a couple of problems.
Firstly, kvm_map_gfn() doesn't work at all for IOMEM pages when the atomic flag is set, which it is in kvm_steal_time_set_preempted(). So a guest vCPU using an IOMEM page for its steal time would never have its preempted field set.
Secondly, the gfn_to_pfn_cache is not invalidated in all cases where it should have been. There are two stages to the GFN->PFN conversion; first the GFN is converted to a userspace HVA, and then that HVA is looked up in the process page tables to find the underlying host PFN. Correct invalidation of the latter would require being hooked up to the MMU notifiers, but that doesn't happen---so it just keeps mapping and unmapping the *wrong* PFN after the userspace page tables change.
In the !IOMEM case at least the stale page *is* pinned all the time it's cached, so it won't be freed and reused by anyone else while still receiving the steal time updates. The map/unmap dance only takes care of the KVM administrivia such as marking the page dirty.
Until the gfn_to_pfn cache handles the remapping automatically by integrating with the MMU notifiers, we might as well not get a kernel mapping of it, and use the perfectly serviceable userspace HVA that we already have. We just need to implement the atomic xchg on the userspace address with appropriate exception handling, which is fairly trivial.
Cc: stable@vger.kernel.org Fixes: b043138246a4 ("x86/KVM: Make sure KVM_VCPU_FLUSH_TLB flag is not missed") Signed-off-by: David Woodhouse dwmw@amazon.co.uk Message-Id: 3645b9b889dac6438394194bb5586a46b68d581f.camel@infradead.org [I didn't entirely agree with David's assessment of the usefulness of the gfn_to_pfn cache, and integrated the outcome of the discussion in the above commit message. - Paolo] 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/x86.c | 105 ++++++++++++++++++++++++++++------------ 2 files changed, 76 insertions(+), 31 deletions(-)
--- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -745,7 +745,7 @@ struct kvm_vcpu_arch { u8 preempted; u64 msr_val; u64 last_steal; - struct gfn_to_pfn_cache cache; + struct gfn_to_hva_cache cache; } st;
u64 l1_tsc_offset; --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3190,8 +3190,11 @@ static void kvm_vcpu_flush_tlb_guest(str
static void record_steal_time(struct kvm_vcpu *vcpu) { - struct kvm_host_map map; - struct kvm_steal_time *st; + struct gfn_to_hva_cache *ghc = &vcpu->arch.st.cache; + struct kvm_steal_time __user *st; + struct kvm_memslots *slots; + u64 steal; + u32 version;
if (kvm_xen_msr_enabled(vcpu->kvm)) { kvm_xen_runstate_set_running(vcpu); @@ -3201,47 +3204,83 @@ static void record_steal_time(struct kvm if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) return;
- /* -EAGAIN is returned in atomic context so we can just return. */ - if (kvm_map_gfn(vcpu, vcpu->arch.st.msr_val >> PAGE_SHIFT, - &map, &vcpu->arch.st.cache, false)) + if (WARN_ON_ONCE(current->mm != vcpu->kvm->mm)) return;
- st = map.hva + - offset_in_page(vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS); + slots = kvm_memslots(vcpu->kvm); + + if (unlikely(slots->generation != ghc->generation || + kvm_is_error_hva(ghc->hva) || !ghc->memslot)) { + gfn_t gfn = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS; + + /* We rely on the fact that it fits in a single page. */ + BUILD_BUG_ON((sizeof(*st) - 1) & KVM_STEAL_VALID_BITS); + + if (kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc, gfn, sizeof(*st)) || + kvm_is_error_hva(ghc->hva) || !ghc->memslot) + return; + } + + st = (struct kvm_steal_time __user *)ghc->hva; + if (!user_access_begin(st, sizeof(*st))) + return;
/* * Doing a TLB flush here, on the guest's behalf, can avoid * expensive IPIs. */ if (guest_pv_has(vcpu, KVM_FEATURE_PV_TLB_FLUSH)) { - u8 st_preempted = xchg(&st->preempted, 0); + u8 st_preempted = 0; + int err = -EFAULT; + + asm volatile("1: xchgb %0, %2\n" + "xor %1, %1\n" + "2:\n" + _ASM_EXTABLE_UA(1b, 2b) + : "+r" (st_preempted), + "+&r" (err) + : "m" (st->preempted)); + if (err) + goto out; + + user_access_end(); + + vcpu->arch.st.preempted = 0;
trace_kvm_pv_tlb_flush(vcpu->vcpu_id, st_preempted & KVM_VCPU_FLUSH_TLB); if (st_preempted & KVM_VCPU_FLUSH_TLB) kvm_vcpu_flush_tlb_guest(vcpu); + + if (!user_access_begin(st, sizeof(*st))) + goto dirty; } else { - st->preempted = 0; + unsafe_put_user(0, &st->preempted, out); + vcpu->arch.st.preempted = 0; }
- vcpu->arch.st.preempted = 0; - - if (st->version & 1) - st->version += 1; /* first time write, random junk */ + unsafe_get_user(version, &st->version, out); + if (version & 1) + version += 1; /* first time write, random junk */
- st->version += 1; + version += 1; + unsafe_put_user(version, &st->version, out);
smp_wmb();
- st->steal += current->sched_info.run_delay - + unsafe_get_user(steal, &st->steal, out); + steal += current->sched_info.run_delay - vcpu->arch.st.last_steal; vcpu->arch.st.last_steal = current->sched_info.run_delay; + unsafe_put_user(steal, &st->steal, out);
- smp_wmb(); - - st->version += 1; + version += 1; + unsafe_put_user(version, &st->version, out);
- kvm_unmap_gfn(vcpu, &map, &vcpu->arch.st.cache, true, false); + out: + user_access_end(); + dirty: + mark_page_dirty_in_slot(vcpu->kvm, ghc->memslot, gpa_to_gfn(ghc->gpa)); }
int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info) @@ -4280,8 +4319,10 @@ void kvm_arch_vcpu_load(struct kvm_vcpu
static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) { - struct kvm_host_map map; - struct kvm_steal_time *st; + struct gfn_to_hva_cache *ghc = &vcpu->arch.st.cache; + struct kvm_steal_time __user *st; + struct kvm_memslots *slots; + static const u8 preempted = KVM_VCPU_PREEMPTED;
if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) return; @@ -4289,16 +4330,23 @@ static void kvm_steal_time_set_preempted if (vcpu->arch.st.preempted) return;
- if (kvm_map_gfn(vcpu, vcpu->arch.st.msr_val >> PAGE_SHIFT, &map, - &vcpu->arch.st.cache, true)) + /* This happens on process exit */ + if (unlikely(current->mm != vcpu->kvm->mm)) return;
- st = map.hva + - offset_in_page(vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS); + slots = kvm_memslots(vcpu->kvm); + + if (unlikely(slots->generation != ghc->generation || + kvm_is_error_hva(ghc->hva) || !ghc->memslot)) + return;
- st->preempted = vcpu->arch.st.preempted = KVM_VCPU_PREEMPTED; + st = (struct kvm_steal_time __user *)ghc->hva; + BUILD_BUG_ON(sizeof(st->preempted) != sizeof(preempted));
- kvm_unmap_gfn(vcpu, &map, &vcpu->arch.st.cache, true, true); + if (!copy_to_user_nofault(&st->preempted, &preempted, sizeof(preempted))) + vcpu->arch.st.preempted = KVM_VCPU_PREEMPTED; + + mark_page_dirty_in_slot(vcpu->kvm, ghc->memslot, gpa_to_gfn(ghc->gpa)); }
void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) @@ -10812,11 +10860,8 @@ void kvm_arch_vcpu_postcreate(struct kvm
void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu) { - struct gfn_to_pfn_cache *cache = &vcpu->arch.st.cache; int idx;
- kvm_release_pfn(cache->pfn, cache->dirty, cache); - kvmclock_reset(vcpu);
static_call(kvm_x86_vcpu_free)(vcpu);