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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Roman Kagan <rkagan@virtuozzo.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>,
        linux-kernel@vger.kernel.org, Jim Mattson <jmattson@google.com>,
        Liran Alon <liran.alon@oracle.com>, kvm@vger.kernel.org
Subject: Re: [PATCH v6 06/13] KVM: nVMX: optimize prepare_vmcs02{,_full} for Enlightened VMCS case
In-Reply-To: <2064be05-4286-e3d5-cd22-13fd957b7003@redhat.com>
References: <20181016165011.6607-1-vkuznets@redhat.com> <20181016165011.6607-7-vkuznets@redhat.com> <2064be05-4286-e3d5-cd22-13fd957b7003@redhat.com>
Date:   Wed, 17 Oct 2018 16:47:27 +0200
Message-ID: <8736t4sk8w.fsf@vitty.brq.redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Paolo Bonzini <pbonzini@redhat.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