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

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Message-ID: <87va93pv6w.fsf@vitty.brq.redhat.com>
Date:   Wed, 25 Jul 2018 10:37:43 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...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

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

> On 21/06/2018 14:30, Vitaly Kuznetsov wrote:
>> +static __always_inline bool evmcs_needs_write(struct vcpu_vmx *vmx,
>> +					      u16 clean_field)
>> +{
>> +	if (vmx->nested.dirty_vmcs12)
>> +		return true;
>
> 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.

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

This series was posted quite some time ago, I'll rebase and re-test with
the fix included.

Thanks!

-- 
  Vitaly