linux-kernel - Re: [RFC PATCH v2 55/69] KVM: VMX: Add 'main.c' to wrap VMX and TDX

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Message-ID: <aca9277a-da65-ab0b-f499-28da1da112e8@redhat.com>
Date:   Thu, 8 Jul 2021 17:29:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     isaku.yamahata@...el.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        kernel test robot <lkp@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [RFC PATCH v2 55/69] KVM: VMX: Add 'main.c' to wrap VMX and TDX

On 08/07/21 17:21, Isaku Yamahata wrote:
> On Tue, Jul 06, 2021 at 04:43:22PM +0200,
> Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
>> On 03/07/21 00:05, isaku.yamahata@...el.com wrote:
>>> +#include "vmx.c"
>>
>> What makes it particularly hard to have this as a separate .o file rather
>> than an #include?
> 
> It's to let complier to optimize functionc call of "if (tdx) tdx_xxx() else vmx_xxx()",
> given x86_ops static call story.

As long as it's not an indirect call, not inlining tdx_xxx and vmx_xxx 
is unlikely to give a lot of benefit.

What you could do, is use a static branch that bypasses the 
"is_tdx_vcpu/vm" check if no TDX VM is running.  A similar technique is 
used to bypass the test for in-kernel APIC if all VM have it.

Paolo