linux-kernel - Re: [PATCH v2 1/8] KVM: x86: Have ____kvm_emulate

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Message-ID: <13d7d1ad-d631-436f-b1b6-a11a77e6660a@intel.com>
Date: Wed, 12 Feb 2025 11:12:15 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>, pbonzini@...hat.com,
 seanjc@...gle.com, kvm@...r.kernel.org
Cc: rick.p.edgecombe@...el.com, kai.huang@...el.com, adrian.hunter@...el.com,
 reinette.chatre@...el.com, tony.lindgren@...el.com,
 isaku.yamahata@...el.com, yan.y.zhao@...el.com, chao.gao@...el.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/8] KVM: x86: Have ____kvm_emulate_hypercall() read
 the GPRs

On 2/12/2025 9:32 AM, Binbin Wu wrote:
> 
> 
> On 2/11/2025 6:23 PM, Xiaoyao Li wrote:
>> On 2/11/2025 10:54 AM, Binbin Wu wrote:
>>> Have ____kvm_emulate_hypercall() read the GPRs instead of passing them
>>> in via the macro.
>>>
>>> When emulating KVM hypercalls via TDVMCALL, TDX will marshall 
>>> registers of
>>> TDVMCALL ABI into KVM's x86 registers to match the definition of KVM
>>> hypercall ABI _before_ ____kvm_emulate_hypercall() gets called. 
>>> Therefore,
>>> ____kvm_emulate_hypercall() can just read registers internally based 
>>> on KVM
>>> hypercall ABI, and those registers can be removed from the
>>> __kvm_emulate_hypercall() macro.
>>>
>>> Also, op_64_bit can be determined inside ____kvm_emulate_hypercall(),
>>> remove it from the __kvm_emulate_hypercall() macro as well.
>>
>> After this patch, __kvm_emulate_hypercall() becomes superfluous.
>> we can just put the logic to call the "complete_hypercall" into 
>> ____kvm_emulate_hypercall() and rename it to __kvm_emulate_hypercall()
>>
>>
> According to the commit message of
> "KVM: x86: Refactor __kvm_emulate_hypercall() into a macro":
> "Rework __kvm_emulate_hypercall() into a macro so that completion of
> hypercalls that don't exit to userspace use direct function calls to the
> completion helper, i.e. don't trigger a retpoline when RETPOLINE=y."

I see.

I thought the purpose of introducing the macro was for TDX usage. My 
fault that didn't checking the commit message of that change.

It makes sense for retpoline reason.

> So I kept the macro.