linux-kernel - Re: [PATCH v2] KVM: X86: Ultra fast single target IPI fastpath

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Message-ID: <f51251cc-885e-2f7a-b18d-faa76db15b87@redhat.com>
Date:   Fri, 10 Apr 2020 17:50:35 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH v2] KVM: X86: Ultra fast single target IPI fastpath

On 10/04/20 17:35, Sean Christopherson wrote:
> IMO, this should come at the very end of vmx_vcpu_run().  At a minimum, it
> needs to be moved below the #MC handling and below
> 
> 	if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> 		return;

Why?  It cannot run in any of those cases, since the vmx->exit_reason
won't match.

> KVM more or less assumes vmx->idt_vectoring_info is always valid, and it's
> not obvious that a generic fastpath call can safely run before
> vmx_complete_interrupts(), e.g. the kvm_clear_interrupt_queue() call.

Not KVM, rather vmx.c.  You're right about a generic fastpath, but in
this case kvm_irq_delivery_to_apic_fast is not touching VMX state; even
if you have a self-IPI, the modification of vCPU state is only scheduled
here and will happen later via either kvm_x86_ops.sync_pir_to_irr or
KVM_REQ_EVENT.

Paolo