From: Sean Christopherson <seanjc@google.com>
To: Xu Yilun <yilun.xu@linux.intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Yan Zhao <yan.y.zhao@intel.com>,
Isaku Yamahata <isaku.yamahata@intel.com>,
Michael Roth <michael.roth@amd.com>,
Yu Zhang <yu.c.zhang@linux.intel.com>,
Chao Peng <chao.p.peng@linux.intel.com>,
Fuad Tabba <tabba@google.com>,
David Matlack <dmatlack@google.com>
Subject: Re: [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above slot validity checks
Date: Mon, 11 Mar 2024 17:08:04 -0700 [thread overview]
Message-ID: <Ze-c5KHDmpz7M6eN@google.com> (raw)
In-Reply-To: <Ze6L/Tnrvs7eayqG@yilunxu-OptiPlex-7050>
On Mon, Mar 11, 2024, Xu Yilun wrote:
> On Fri, Mar 08, 2024 at 03:28:08PM -0800, Sean Christopherson wrote:
> > On Fri, Mar 08, 2024, Xu Yilun wrote:
> > > On Tue, Feb 27, 2024 at 06:41:40PM -0800, Sean Christopherson wrote:
> > > > @@ -4410,6 +4405,16 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> > > > fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
> > > > smp_rmb();
> > > >
> > > > + /*
> > > > + * Check for a private vs. shared mismatch *after* taking a snapshot of
> > > > + * mmu_invalidate_seq, as changes to gfn attributes are guarded by the
> > > > + * invalidation notifier.
> > >
> > > I didn't see how mmu_invalidate_seq influences gfn attribute judgement.
> > > And there is no synchronization between the below check and
> > > kvm_vm_set_mem_attributes(), the gfn attribute could still be changing
> > > after the snapshot.
> >
> > There is synchronization. If kvm_vm_set_mem_attributes() changes the attributes,
> > and thus bumps mmu_invalidate_seq, after kvm_faultin_pfn() takes its snapshot,
> > then is_page_fault_stale() will detect that an invalidation related to the gfn
> > occured and resume the guest *without* installing a mapping in KVM's page tables.
> >
> > I.e. KVM may read the old, stale gfn attributes, but it will never actually
> > expose the stale attirubtes to the guest.
>
> That makes sense! I was just thinking of the racing for below few lines,
>
> if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> return -EFAULT;
> }
>
> But the guarding is actually for the whole kvm_faultin_pfn(). It is the
> the same mechanism between getting old gfn attributes and getting old pfn.
>
> I wonder if we could instead add some general comments at
>
> fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
>
> about the snapshot and is_page_fault_stale() thing.
Yeah, I'll add a comment. The only reason not to add a comment is that, ideally,
the comment/documentation would live in common KVM code, not x86. But this code
already has a few big comments about the mmu_notifier retry logic, one more
definitely won't hurt.
next prev parent reply other threads:[~2024-03-12 0:08 UTC|newest]
Thread overview: 83+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-28 2:41 [PATCH 00/16] KVM: x86/mmu: Page fault and MMIO cleanups Sean Christopherson
2024-02-28 2:41 ` [PATCH 01/16] KVM: x86/mmu: Exit to userspace with -EFAULT if private fault hits emulation Sean Christopherson
2024-03-01 8:48 ` Xiaoyao Li
2024-03-07 12:52 ` Gupta, Pankaj
2024-03-12 2:59 ` Binbin Wu
2024-04-04 16:38 ` Sean Christopherson
2024-03-08 4:22 ` Yan Zhao
2024-04-04 16:45 ` Sean Christopherson
2024-02-28 2:41 ` [PATCH 02/16] KVM: x86: Remove separate "bit" defines for page fault error code masks Sean Christopherson
2024-02-29 12:44 ` Paolo Bonzini
2024-02-29 18:40 ` Sean Christopherson
2024-02-29 20:56 ` Paolo Bonzini
2024-02-29 13:43 ` Dongli Zhang
2024-02-29 15:25 ` Sean Christopherson
2024-02-28 2:41 ` [PATCH 03/16] KVM: x86: Define more SEV+ page fault error bits/flags for #NPF Sean Christopherson
2024-02-28 4:43 ` Dongli Zhang
2024-02-28 16:16 ` Sean Christopherson
2024-02-28 2:41 ` [PATCH 04/16] KVM: x86/mmu: Pass full 64-bit error code when handling page faults Sean Christopherson
2024-02-28 7:30 ` Dongli Zhang
2024-02-28 16:22 ` Sean Christopherson
2024-02-29 13:32 ` Dongli Zhang
2024-03-05 3:55 ` Xiaoyao Li
2024-02-28 2:41 ` [PATCH 05/16] KVM: x86/mmu: Use synthetic page fault error code to indicate private faults Sean Christopherson
2024-02-29 11:16 ` Huang, Kai
2024-02-29 15:17 ` Sean Christopherson
2024-03-06 9:43 ` Xu Yilun
2024-03-06 14:45 ` Sean Christopherson
2024-03-07 9:05 ` Xu Yilun
2024-03-07 14:36 ` Sean Christopherson
2024-03-12 5:34 ` Binbin Wu
2024-02-28 2:41 ` [PATCH 06/16] KVM: x86/mmu: WARN if upper 32 bits of legacy #PF error code are non-zero Sean Christopherson
2024-02-29 22:11 ` Huang, Kai
2024-02-29 23:07 ` Sean Christopherson
2024-03-12 5:44 ` Binbin Wu
2024-02-28 2:41 ` [PATCH 07/16] KVM: x86: Move synthetic PFERR_* sanity checks to SVM's #NPF handler Sean Christopherson
2024-02-29 22:19 ` Huang, Kai
2024-02-29 22:52 ` Sean Christopherson
2024-02-29 23:14 ` Huang, Kai
2024-03-12 9:44 ` Binbin Wu
2024-02-28 2:41 ` [PATCH 08/16] KVM: x86/mmu: WARN and skip MMIO cache on private, reserved page faults Sean Christopherson
2024-02-29 22:26 ` Huang, Kai
2024-02-29 23:06 ` Sean Christopherson
2024-02-29 23:21 ` Huang, Kai
2024-03-04 15:51 ` Sean Christopherson
2024-03-05 21:32 ` Huang, Kai
2024-03-06 0:25 ` Sean Christopherson
2024-02-28 2:41 ` [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above slot validity checks Sean Christopherson
2024-03-05 23:06 ` Huang, Kai
2024-03-06 0:38 ` Sean Christopherson
2024-03-06 1:22 ` Huang, Kai
2024-03-06 2:02 ` Sean Christopherson
2024-03-06 22:06 ` Huang, Kai
2024-03-06 23:49 ` Sean Christopherson
2024-03-07 0:28 ` Huang, Kai
2024-03-08 4:54 ` Xu Yilun
2024-03-08 23:28 ` Sean Christopherson
2024-03-11 4:43 ` Xu Yilun
2024-03-12 0:08 ` Sean Christopherson [this message]
2024-02-28 2:41 ` [PATCH 10/16] KVM: x86/mmu: Don't force emulation of L2 accesses to non-APIC internal slots Sean Christopherson
2024-03-07 0:03 ` Huang, Kai
2024-02-28 2:41 ` [PATCH 11/16] KVM: x86/mmu: Explicitly disallow private accesses to emulated MMIO Sean Christopherson
2024-03-06 22:35 ` Huang, Kai
2024-03-06 22:43 ` Sean Christopherson
2024-03-06 22:49 ` Huang, Kai
2024-03-06 23:01 ` Sean Christopherson
2024-03-06 23:20 ` Huang, Kai
2024-03-07 17:10 ` Kirill A. Shutemov
2024-03-08 0:09 ` Huang, Kai
2024-02-28 2:41 ` [PATCH 12/16] KVM: x86/mmu: Move slot checks from __kvm_faultin_pfn() to kvm_faultin_pfn() Sean Christopherson
2024-03-07 0:11 ` Huang, Kai
2024-02-28 2:41 ` [PATCH 13/16] KVM: x86/mmu: Handle no-slot faults at the beginning of kvm_faultin_pfn() Sean Christopherson
2024-03-07 0:48 ` Huang, Kai
2024-03-07 0:53 ` Sean Christopherson
2024-02-28 2:41 ` [PATCH 14/16] KVM: x86/mmu: Set kvm_page_fault.hva to KVM_HVA_ERR_BAD for "no slot" faults Sean Christopherson
2024-03-07 0:50 ` Huang, Kai
2024-03-07 1:01 ` Sean Christopherson
2024-02-28 2:41 ` [PATCH 15/16] KVM: x86/mmu: Initialize kvm_page_fault's pfn and hva to error values Sean Christopherson
2024-03-07 0:46 ` Huang, Kai
2024-02-28 2:41 ` [PATCH 16/16] KVM: x86/mmu: Sanity check that __kvm_faultin_pfn() doesn't create noslot pfns Sean Christopherson
2024-03-07 0:46 ` Huang, Kai
2024-04-17 12:48 ` [PATCH 00/16] KVM: x86/mmu: Page fault and MMIO cleanups Paolo Bonzini
2024-04-18 15:40 ` Sean Christopherson
2024-04-19 6:47 ` Xiaoyao Li
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