linux-kernel - Re: [PATCH 06/10] KVM: x86: Move "flush guest's TLB" logic to separate kvm_x86

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Message-ID: <ca7bcbc2-d7eb-bc7c-4e15-a77831af8a73@redhat.com>
Date:   Fri, 21 Feb 2020 18:32:48 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/10] KVM: x86: Move "flush guest's TLB" logic to
 separate kvm_x86_ops hook

On 20/02/20 21:43, Sean Christopherson wrote:
> Add a dedicated hook to handle flushing TLB entries on behalf of the
> guest, i.e. for a paravirtualized TLB flush, and use it directly instead
> of bouncing through kvm_vcpu_flush_tlb().  Change the effective VMX
> implementation to never do INVEPT, i.e. to always flush via INVVPID.
> The INVEPT performed by __vmx_flush_tlb() when @invalidate_gpa=false and
> enable_vpid=0 is unnecessary, as it will only flush GPA->HPA mappings;
> GVA->GPA and GVA->HPA translations are flushed by VM-Enter when VPID is
> disabled, and changes in the guest pages tables only affect GVA->*PA
> mappings.
> 
> When EPT and VPID are enabled, doing INVVPID is not required (by Intel's
> architecture) to invalidate GPA mappings, i.e. TLB entries that cache
> GPA->HPA translations can live across INVVPID as GPA->HPA mappings are
> associated with an EPTP, not a VPID.  The intent of @invalidate_gpa is
> to inform vmx_flush_tlb() that it needs to "invalidate gpa mappings",
> i.e. do INVEPT and not simply INVVPID.  Other than nested VPID handling,
> which now calls vpid_sync_context() directly, the only scenario where
> KVM can safely do INVVPID instead of INVEPT (when EPT is enabled) is if
> KVM is flushing TLB entries from the guest's perspective, i.e. is
> invalidating GLA->GPA mappings.

Since you need a v2, can you replace the name of mappings with "linear",
"guest-physical" and "combined" as in the SDM?  It takes a little to get
used to them but it avoids three-letter acronym soup.

Paolo