| Message ID | 0dabeeb789f57b0d793f85d073893063e692032d.1647336064.git.houwenlong.hwl@antgroup.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86/mmu: Don't rebuild page when the page is synced and no tlb flushing is required | expand |
On Tue, Mar 15, 2022 at 6:45 PM Hou Wenlong <houwenlong.hwl@antgroup.com> wrote: > > Before Commit c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() > to return true when remote flush is needed"), the return value > of kvm_sync_page() indicates whether the page is synced, and > kvm_mmu_get_page() would rebuild page when the sync fails. > But now, kvm_sync_page() returns false when the page is > synced and no tlb flushing is required, which leads to > rebuild page in kvm_mmu_get_page(). So return the return > value of mmu->sync_page() directly and check it in > kvm_mmu_get_page(). If the sync fails, the page will be > zapped and the invalid_list is not empty, so set flush as > true is accepted in mmu_sync_children(). > Good catch. Acked-by: Lai Jiangshan <jiangshanlai@gmail.com> > Fixes: c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed") > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com> > --- > arch/x86/kvm/mmu/mmu.c | 16 ++++++++-------- > 1 file changed, 8 insertions(+), 8 deletions(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 3b8da8b0745e..8efd165ee27c 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -1866,17 +1866,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm, > &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)]) \ > if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else > > -static bool kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, > +static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, > struct list_head *invalid_list) The comments for FNAME(sync_page) can be copied here.
On Tue, Mar 15, 2022, Hou Wenlong wrote: > Before Commit c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() > to return true when remote flush is needed"), the return value > of kvm_sync_page() indicates whether the page is synced, and > kvm_mmu_get_page() would rebuild page when the sync fails. > But now, kvm_sync_page() returns false when the page is > synced and no tlb flushing is required, which leads to > rebuild page in kvm_mmu_get_page(). So return the return > value of mmu->sync_page() directly and check it in > kvm_mmu_get_page(). If the sync fails, the page will be > zapped and the invalid_list is not empty, so set flush as > true is accepted in mmu_sync_children(). > > Fixes: c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed") > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com> > --- > arch/x86/kvm/mmu/mmu.c | 16 ++++++++-------- > 1 file changed, 8 insertions(+), 8 deletions(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 3b8da8b0745e..8efd165ee27c 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -1866,17 +1866,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm, > &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)]) \ > if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else > > -static bool kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, > +static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, > struct list_head *invalid_list) > { > int ret = vcpu->arch.mmu->sync_page(vcpu, sp); > > - if (ret < 0) { > + if (ret < 0) > kvm_mmu_prepare_zap_page(vcpu->kvm, sp, invalid_list); > - return false; > - } > - > - return !!ret; > + return ret; Hrm, this creates an oddity in mmu_sync_children(), which does a logical-OR of the result into a boolean. It doesn't actually change the functionality since kvm_mmu_remote_flush_or_zap() will prioritize invalid_list, but it's weird. What about checking invalid_list directly and keeping the boolean return? Compile tested only. From: Sean Christopherson <seanjc@google.com> Date: Thu, 24 Mar 2022 13:07:32 -0700 Subject: [PATCH] KVM: x86/mmu: Fix shadow reuse when unsync sp doesn't need TLB flush Use invalid_list to detect if synchronizing an unsync shadow page failed and resulted in the page being zapped. Since commit c3e5e415bc1e ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed"), kvm_sync_page() returns whether or not a TLB flush is required, it doesn't provide any indication as to whether or not the sync was successful. If the sync is successful but doesn't require a TLB flush, checking the TLB flush result will cause KVM to unnecessarily rebuild the shadow page. Fixes: c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed") Cc: stable@vger.kernel.org Reported-by: Hou Wenlong <houwenlong.hwl@antgroup.com> Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kvm/mmu/mmu.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 1361eb4599b4..b6350fec1b11 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -2086,16 +2086,19 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, * This way the validity of the mapping is ensured, but the * overhead of write protection is not incurred until the * guest invalidates the TLB mapping. This allows multiple - * SPs for a single gfn to be unsync. - * + * SPs for a single gfn to be unsync. kvm_sync_page() + * returns true if a TLB flush is needed to ensure the + * guest sees the synchronized shadow page. + */ + if (kvm_sync_page(vcpu, sp, &invalid_list)) + kvm_flush_remote_tlbs(vcpu->kvm); + + /* * If the sync fails, the page is zapped. If so, break * in order to rebuild it. */ - if (!kvm_sync_page(vcpu, sp, &invalid_list)) + if (!list_empty(&invalid_list)) break; - - WARN_ON(!list_empty(&invalid_list)); - kvm_flush_remote_tlbs(vcpu->kvm); } __clear_sp_write_flooding_count(sp); base-commit: 9b6a3be37eacee49a659232e86019e733597c045 --
> Before Commit c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() > to return true when remote flush is needed"), the return value > of kvm_sync_page() indicates whether the page is synced, and > kvm_mmu_get_page() would rebuild page when the sync fails. > But now, kvm_sync_page() returns false when the page is > synced and no tlb flushing is required, which leads to > rebuild page in kvm_mmu_get_page(). So return the return > value of mmu->sync_page() directly and check it in > kvm_mmu_get_page(). If the sync fails, the page will be > zapped and the invalid_list is not empty, so set flush as > true is accepted in mmu_sync_children(). > > Fixes: c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed") > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com> Queued and Cc'ed to stable@, thanks. Paolo
On 3/24/22 21:17, Sean Christopherson wrote: >> +static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, >> struct list_head *invalid_list) >> { >> int ret = vcpu->arch.mmu->sync_page(vcpu, sp); >> >> - if (ret < 0) { >> + if (ret < 0) >> kvm_mmu_prepare_zap_page(vcpu->kvm, sp, invalid_list); >> - return false; >> - } >> - >> - return !!ret; >> + return ret; > Hrm, this creates an oddity in mmu_sync_children(), which does a logical-OR of > the result into a boolean. It doesn't actually change the functionality since > kvm_mmu_remote_flush_or_zap() will prioritize invalid_list, but it's weird. > > What about checking invalid_list directly and keeping the boolean return? Compile > tested only. It's even better to check flush |= kvm_sync_page(vcpu, sp, &invalid_list) > 0; in mmu_sync_children. If the returned value is <0, then the page is added to invalid_list and there is no need to set flush = true, just like there is no need to call kvm_flush_remote_tlbs() in kvm_mmu_get_page(). Paolo
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 3b8da8b0745e..8efd165ee27c 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -1866,17 +1866,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm, &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)]) \ if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else -static bool kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, +static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, struct list_head *invalid_list) { int ret = vcpu->arch.mmu->sync_page(vcpu, sp); - if (ret < 0) { + if (ret < 0) kvm_mmu_prepare_zap_page(vcpu->kvm, sp, invalid_list); - return false; - } - - return !!ret; + return ret; } static bool kvm_mmu_remote_flush_or_zap(struct kvm *kvm, @@ -2039,6 +2036,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, struct hlist_head *sp_list; unsigned quadrant; struct kvm_mmu_page *sp; + int ret; int collisions = 0; LIST_HEAD(invalid_list); @@ -2091,11 +2089,13 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, * If the sync fails, the page is zapped. If so, break * in order to rebuild it. */ - if (!kvm_sync_page(vcpu, sp, &invalid_list)) + ret = kvm_sync_page(vcpu, sp, &invalid_list); + if (ret < 0) break; WARN_ON(!list_empty(&invalid_list)); - kvm_flush_remote_tlbs(vcpu->kvm); + if (ret > 0) + kvm_flush_remote_tlbs(vcpu->kvm); } __clear_sp_write_flooding_count(sp);
Before Commit c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed"), the return value of kvm_sync_page() indicates whether the page is synced, and kvm_mmu_get_page() would rebuild page when the sync fails. But now, kvm_sync_page() returns false when the page is synced and no tlb flushing is required, which leads to rebuild page in kvm_mmu_get_page(). So return the return value of mmu->sync_page() directly and check it in kvm_mmu_get_page(). If the sync fails, the page will be zapped and the invalid_list is not empty, so set flush as true is accepted in mmu_sync_children(). Fixes: c3e5e415bc1e6 ("KVM: X86: Change kvm_sync_page() to return true when remote flush is needed") Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com> --- arch/x86/kvm/mmu/mmu.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-)