linux-kernel - Re: [PATCH v6 06/13] KVM: nVMX: optimize prepare_vmcs02{,

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Date:   Wed, 17 Oct 2018 16:47:27 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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>,
        linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Liran Alon <liran.alon@...cle.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v6 06/13] KVM: nVMX: optimize prepare_vmcs02{,_full} for Enlightened VMCS case

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 16/10/2018 18:50, Vitaly Kuznetsov wrote:
>> +	if (!hv_evmcs || !(hv_evmcs->hv_clean_fields &
>> +			   HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP2)) {
>> +		vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector);
>> +		vmcs_write32(GUEST_CS_LIMIT, vmcs12->guest_cs_limit);
>> +		vmcs_write32(GUEST_CS_AR_BYTES, vmcs12->guest_cs_ar_bytes);
>> +		vmcs_writel(GUEST_ES_BASE, vmcs12->guest_es_base);
>> +		vmcs_writel(GUEST_CS_BASE, vmcs12->guest_cs_base);
>> +	}
>
> For what it's worth, I suspect that these can be moved to
> prepare_vmcs02_full.  The initial implementation of shadow VMCS did not
> expose "unrestricted guest" to the L1 hypervisor, and emulation does a
> lot of accesses to CS (of course).  Not sure how ES base ended up in
> there and not DS base, though...

I tried unshadowing all these fields and at least Hyper-V on KVM
(without using eVMCS of course) experiences a 1200-1300 cpu cycles
regression during tight cpuid loop test. I checked and this happens
because it likes vmreading GUEST_CS_AR_BYTES a lot.

-- 
Vitaly