linux-kernel - Re: [PATCH v2 6/6] KVM: nVMX: optimize prepare_vmcs02{,

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Date:   Wed, 25 Jul 2018 14:14:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, linux-kernel@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 6/6] KVM: nVMX: optimize prepare_vmcs02{,_full} for
 Enlightened VMCS case

On 25/07/2018 10:37, Vitaly Kuznetsov wrote:
>> Why is this needed?  If it weren't for it, you could pass hv_evmcs
>> directly to evmcs_needs_write, which would simplify the code a bit in
>> the caller.
> This is an equivalent of prepare_vmcs02()/prepare_vmcs02_full() split
> for eVMCS case: when we switch from L2 guest A to L2 guest B we need to
> write the whole VMCS so evmcs_needs_write() needs to return true.

Right, I missed the dirty_vmcs12 assignment in patch 5.

But is L0 allowed to write to hv_clean_fields?  One possibility is to
add a dirty_evmcs field to struct nested_vmx, and "OR" ~hv_clean_fields
into it at the beginning of prepare_vmcs02.

Something like

	if (vmx->nested.hv_evmcs) {
		vmx->nested.dirty_evmcs |=
			~vmx->nested.hv_evmcs->hv_clean_fields;
		prepare_vmcs02_full(vcpu, vmcs12,
				    vmx->nested.dirty_evmcs);
	} else if (vmx->nested.dirty_vmcs12) {
		prepare_vmcs02_full(vcpu, vmcs12, ~0);
	}

	...
	vmx->nested.dirty_evmcs = 0;
	vmx->nested.dirty_vmcs12 = false;

?

Paolo

> We can, however, make an optimisation: forcefuly reset hv_clean_fields
> mask on enlightened vmptrld making 'dirty_vmcs12' check redundant.