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From: Lai Jiangshan <jiangshanlai+lkml@gmail.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 01/37] KVM: VMX: Flush all EPTP/VPID contexts on remote TLB flush
Date: Wed, 4 Aug 2021 11:11:52 +0800	[thread overview]
Message-ID: <CAJhGHyDbCbP3+oN-EpX_KLYKpzDhotpwASAxMSRScGjtdRNOtA@mail.gmail.com> (raw)
In-Reply-To: <YQljNBBp/EousNBk@google.com>

On Tue, Aug 3, 2021 at 11:39 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Aug 03, 2021, Lai Jiangshan wrote:
> > (I'm replying to a very old email, so many CCs are dropped.)
> >
> > On Sat, Mar 21, 2020 at 5:33 AM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> > > Flush all EPTP/VPID contexts if a TLB flush _may_ have been triggered by
> > > a remote or deferred TLB flush, i.e. by KVM_REQ_TLB_FLUSH.  Remote TLB
> > > flushes require all contexts to be invalidated, not just the active
> > > contexts, e.g. all mappings in all contexts for a given HVA need to be
> > > invalidated on a mmu_notifier invalidation.  Similarly, the instigator
> > > of the deferred TLB flush may be expecting all contexts to be flushed,
> > > e.g. vmx_vcpu_load_vmcs().
> > >
> > > Without nested VMX, flushing only the current EPTP/VPID context isn't
> > > problematic because KVM uses a constant VPID for each vCPU, and
> >
> > Hello, Sean
> >
> > Is the patch optimized for cases where nested VMX is active?
>
> Well, this patch isn't, but KVM has since been optimized to do full EPT/VPID
> flushes only when "necessary".  Necessary in quotes because the two uses can
> technically be further optimized, but doing so would incur significant complexity.

Hello, thanks for your reply.

I know there might be a lot of possible optimizations to be considered, many of
which are too complicated to be implemented.

The optimization I considered yesterday is "ept_sync_global() V.S.
ept_sync_context(this_vcpu's)" in the case: when the VM is using EPT and
doesn't allow nested VMs.  (And I failed to express it yesterday)

In this case, the vCPU uses only one single root_hpa, and I think ept sync
for single context is enough for both cases you listed below.

When the context is flushed, the TLB for the vCPU is clean to run.

If kvm changes the mmu->root_hpa, it is kvm's responsibility to request
another flush which is implemented.

In other words, KVM_REQ_TLB_FLUSH == KVM_REQ_TLB_FLUSH_CURRENT in this case.
And before this patch, kvm flush only the single context rather than global.

>
> Use #1 is remote flushes from the MMU, which don't strictly require a global flush,
> but KVM would need to propagate more information (mmu_role?) in order for responding
> vCPUs to determine what contexts needs to be flushed.  And practically speaking,
> for MMU flushes there's no meaningful difference when using TDP without nested
> guests as the common case will be that each vCPU has a single active EPTP and
> that EPTP will be affected by the MMU changes, i.e. needs to be flushed.

I don't see when we need "to determine what contexts" since the vcpu is
using only one context in this case which is the assumption in my mind,
could you please correct me if I'm wrong.

Thanks,
Lai.

>
> Use #2 is in VMX's pCPU migration path.  Again, not strictly necessary as KVM could
> theoretically track which pCPUs have run a particular vCPU and when that pCPU last
> flushed EPT contexts, but fully solving the problem would be quite complex.  Since
> pCPU migration is always going to be a slow path, the extra complexity would be
> very difficult to justify.
>
> > I think the non-nested cases are normal cases.
> >
> > Although the related code has been changed, the logic of the patch
> > is still working now, would it be better if we restore the optimization
> > for the normal cases (non-nested)?
>
> As above, vmx_flush_tlb_all() hasn't changed, but the callers have.

  reply	other threads:[~2021-08-04  3:12 UTC|newest]

Thread overview: 83+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-20 21:27 [PATCH v3 00/37] KVM: x86: TLB flushing fixes and enhancements Sean Christopherson
2020-03-20 21:27 ` [PATCH v3 01/37] KVM: VMX: Flush all EPTP/VPID contexts on remote TLB flush Sean Christopherson
2021-08-03  1:45   ` Lai Jiangshan
2021-08-03 15:39     ` Sean Christopherson
2021-08-04  3:11       ` Lai Jiangshan [this message]
2021-08-04 15:33         ` Sean Christopherson
2020-03-20 21:27 ` [PATCH v3 02/37] KVM: nVMX: Validate the EPTP when emulating INVEPT(EXTENT_CONTEXT) Sean Christopherson
2020-03-23 14:51   ` Vitaly Kuznetsov
2020-03-23 15:45     ` Sean Christopherson
2020-03-23 23:46       ` Paolo Bonzini
2020-03-20 21:27 ` [PATCH v3 03/37] KVM: nVMX: Invalidate all EPTP contexts when emulating INVEPT for L1 Sean Christopherson
2020-03-23 15:24   ` Vitaly Kuznetsov
2020-03-23 15:53     ` Sean Christopherson
2020-03-23 16:24   ` Jim Mattson
2020-03-23 16:28     ` Sean Christopherson
2020-03-23 16:36       ` Jim Mattson
2020-03-23 16:44         ` Sean Christopherson
2020-03-23 23:50           ` Paolo Bonzini
2020-03-24  0:12             ` Jim Mattson
2020-03-30 18:38               ` Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 04/37] KVM: nVMX: Invalidate all roots when emulating INVVPID without EPT Sean Christopherson
2020-03-23 15:34   ` Vitaly Kuznetsov
2020-03-23 16:04     ` Sean Christopherson
2020-03-23 16:33       ` Vitaly Kuznetsov
2020-03-23 16:50         ` Sean Christopherson
2020-03-23 16:57           ` Vitaly Kuznetsov
2020-03-20 21:28 ` [PATCH v3 05/37] KVM: x86: Export kvm_propagate_fault() (as kvm_inject_emulated_page_fault) Sean Christopherson
2020-03-23 15:47   ` Vitaly Kuznetsov
2020-03-23 16:24     ` Sean Christopherson
2020-03-23 23:56       ` Paolo Bonzini
2020-03-20 21:28 ` [PATCH v3 06/37] KVM: x86: Consolidate logic for injecting page faults to L1 Sean Christopherson
2020-03-24  0:47   ` Paolo Bonzini
2020-03-20 21:28 ` [PATCH v3 07/37] KVM: x86: Sync SPTEs when injecting page/EPT fault into L1 Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 08/37] KVM: VMX: Skip global INVVPID fallback if vpid==0 in vpid_sync_context() Sean Christopherson
2020-03-25  9:33   ` Vitaly Kuznetsov
2020-03-20 21:28 ` [PATCH v3 09/37] KVM: VMX: Use vpid_sync_context() directly when possible Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 10/37] KVM: VMX: Move vpid_sync_vcpu_addr() down a few lines Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 11/37] KVM: VMX: Handle INVVPID fallback logic in vpid_sync_vcpu_addr() Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 12/37] KVM: VMX: Drop redundant capability checks in low level INVVPID helpers Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 13/37] KVM: nVMX: Use vpid_sync_vcpu_addr() to emulate INVVPID with address Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 14/37] KVM: x86: Move "flush guest's TLB" logic to separate kvm_x86_ops hook Sean Christopherson
2020-03-25 10:23   ` Vitaly Kuznetsov
2020-03-25 15:41     ` Paolo Bonzini
2020-03-25 16:08       ` Vitaly Kuznetsov
2020-03-25 15:48     ` Sean Christopherson
2020-03-25 16:11       ` Vitaly Kuznetsov
2020-03-20 21:28 ` [PATCH v3 15/37] KVM: VMX: Clean up vmx_flush_tlb_gva() Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 16/37] KVM: x86: Drop @invalidate_gpa param from kvm_x86_ops' tlb_flush() Sean Christopherson
2020-03-25 11:23   ` Vitaly Kuznetsov
2020-03-20 21:28 ` [PATCH v3 17/37] KVM: SVM: Wire up ->tlb_flush_guest() directly to svm_flush_tlb() Sean Christopherson
2020-03-25 11:23   ` Vitaly Kuznetsov
2020-03-20 21:28 ` [PATCH v3 18/37] KVM: VMX: Move vmx_flush_tlb() to vmx.c Sean Christopherson
2020-03-25 11:25   ` Vitaly Kuznetsov
2020-03-20 21:28 ` [PATCH v3 19/37] KVM: nVMX: Move nested_get_vpid02() to vmx/nested.h Sean Christopherson
2020-03-25 11:25   ` Vitaly Kuznetsov
2020-03-20 21:28 ` [PATCH v3 20/37] KVM: VMX: Introduce vmx_flush_tlb_current() Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 21/37] KVM: SVM: Document the ASID logic in svm_flush_tlb() Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 22/37] KVM: x86: Rename ->tlb_flush() to ->tlb_flush_all() Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 23/37] KVM: nVMX: Add helper to handle TLB flushes on nested VM-Enter/VM-Exit Sean Christopherson
2021-10-28 13:11   ` Lai Jiangshan
2021-10-28 15:22     ` Sean Christopherson
2021-10-29  0:44       ` Lai Jiangshan
2021-10-29 17:10         ` Sean Christopherson
2021-10-30  1:34           ` Lai Jiangshan
2021-11-04 17:47             ` Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 24/37] KVM: x86: Introduce KVM_REQ_TLB_FLUSH_CURRENT to flush current ASID Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 25/37] KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 26/37] KVM: nVMX: Selectively use TLB_FLUSH_CURRENT for nested VM-Enter/VM-Exit Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 27/37] KVM: nVMX: Reload APIC access page on nested VM-Exit only if necessary Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 28/37] KVM: VMX: Retrieve APIC access page HPA only when necessary Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 29/37] KVM: VMX: Don't reload APIC access page if its control is disabled Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 30/37] KVM: x86/mmu: Move fast_cr3_switch() side effects to __kvm_mmu_new_cr3() Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 31/37] KVM: x86/mmu: Add separate override for MMU sync during fast CR3 switch Sean Christopherson
2020-03-24 11:07   ` Paolo Bonzini
2020-03-20 21:28 ` [PATCH v3 32/37] KVM: x86/mmu: Add module param to force TLB flush on root reuse Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 33/37] KVM: nVMX: Skip MMU sync on nested VMX transition when possible Sean Christopherson
2020-03-24 11:19   ` Paolo Bonzini
2020-03-20 21:28 ` [PATCH v3 34/37] KVM: nVMX: Don't flush TLB on nested VMX transition Sean Christopherson
2020-03-24 11:20   ` Paolo Bonzini
2020-03-24 18:10     ` Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 35/37] KVM: nVMX: Free only the affected contexts when emulating INVEPT Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 36/37] KVM: x86: Replace "cr3" with "pgd" in "new cr3/pgd" related code Sean Christopherson
2020-03-20 21:28 ` [PATCH v3 37/37] KVM: VMX: Clean cr3/pgd handling in vmx_load_mmu_pgd() Sean Christopherson

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