linux-kernel - Re: [PATCH v2 23/32] KVM: nVMX: Add helper to handle TLB flushes on nested VM-Enter/VM-Exit

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Message-ID: <f4850521-29fe-51ff-05e7-76cef1fa0fd9@redhat.com>
Date:   Wed, 18 Mar 2020 11:45:57 +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, Ben Gardon <bgardon@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 23/32] KVM: nVMX: Add helper to handle TLB flushes on
 nested VM-Enter/VM-Exit

On 17/03/20 19:18, Sean Christopherson wrote:
> On Tue, Mar 17, 2020 at 06:17:59PM +0100, Paolo Bonzini wrote:
>> On 17/03/20 05:52, Sean Christopherson wrote:
>>> +	nested_vmx_transition_tlb_flush(vcpu, vmcs12);
>>> +
>>> +	/*
>>> +	 * There is no direct mapping between vpid02 and vpid12, vpid02 is
>>> +	 * per-vCPU and reused for all nested vCPUs.  If vpid12 is changing
>>> +	 * then the new "virtual" VPID will reuse the same "real" VPID,
>>> +	 * vpid02, and so needs to be sync'd.  Skip the sync if a TLB flush
>>> +	 * has already been requested, but always update the last used VPID.
>>> +	 */
>>> +	if (nested_cpu_has_vpid(vmcs12) && nested_has_guest_tlb_tag(vcpu) &&
>>> +	    vmcs12->virtual_processor_id != vmx->nested.last_vpid) {
>>> +		vmx->nested.last_vpid = vmcs12->virtual_processor_id;
>>> +		if (!kvm_test_request(KVM_REQ_TLB_FLUSH, vcpu))
>>> +			vpid_sync_context(nested_get_vpid02(vcpu));
>>>  	}
>>
>> Would it make sense to move nested_vmx_transition_tlb_flush into an
>> "else" branch?
> 
> Maybe?  I tried that at one point, but didn't like making the call to
> nested_vmx_transition_tlb_flush() conditional.  My intent is to have
> the ...tlb_flush() call be standalone, i.e. logic that is common to all
> nested transitions, so that someone can look at the code can easily
> (relatively speaking) understand the basic rules for TLB flushing on
> nested transitions.

I think it's clear from the above code that we're handling a TLB flush
in a way that doesn't require nested_vmx_transition_tlb_flush.  But
perhaps I didn't understand what you mean by "logic that is common to
all nested transitions" and why you named it
nested_vmx_transition_tlb_flush.

Perhaps nested_vmx_transition_tlb_flush could grow a vmentry/vmexit bool
argument instead?

> I also tried the oppositie, i.e. putting the above code in an else-branch,
> with nested_vmx_transition_tlb_flush() returning true if it requested a
> flush.  But that required updating vmx->nested.last_vpid in a separate
> flow, which was quite awkward.

No, that's awkward indeed.

Paolo

>> And should this also test that KVM_REQ_TLB_FLUSH_CURRENT is not set?
> 
> Doh, yes.
>