Re: [PATCH v6 06/13] KVM: nVMX: optimize prepare_vmcs02{,_full} for Enlightened VMCS case

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <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
Date: Wed, 17 Oct 2018 16:47:27 +0200	[thread overview]
Message-ID: <8736t4sk8w.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <2064be05-4286-e3d5-cd22-13fd957b7003@redhat.com>

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