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Date: Wed, 3 Apr 2024 18:15:45 -0700
From: Isaku Yamahata <isaku.yamahata@intel.com>
To: Chao Gao <chao.gao@intel.com>
Cc: isaku.yamahata@intel.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, isaku.yamahata@gmail.com,
	Paolo Bonzini <pbonzini@redhat.com>, erdemaktas@google.com,
	Sean Christopherson <seanjc@google.com>,
	Sagi Shahar <sagis@google.com>, Kai Huang <kai.huang@intel.com>,
	chen.bo@intel.com, hang.yuan@intel.com, tina.zhang@intel.com,
	Xiaoyao Li <xiaoyao.li@intel.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	isaku.yamahata@linux.intel.com
Subject: Re: [PATCH v19 104/130] KVM: TDX: Add a place holder for handler of
 TDX hypercalls (TDG.VP.VMCALL)
Message-ID: <20240404011545.GO2444378@ls.amr.corp.intel.com>
References: <cover.1708933498.git.isaku.yamahata@intel.com>
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 <ZgqFhx7JjhzKXjqb@chao-email>
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In-Reply-To: <ZgqFhx7JjhzKXjqb@chao-email>

On Mon, Apr 01, 2024 at 05:59:35PM +0800,
Chao Gao <chao.gao@intel.com> wrote:

> > static inline bool is_td_vcpu_created(struct vcpu_tdx *tdx)
> > {
> > 	return tdx->td_vcpu_created;
> >@@ -897,6 +932,11 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu)
> > 
> > 	tdx_complete_interrupts(vcpu);
> > 
> >+	if (tdx->exit_reason.basic == EXIT_REASON_TDCALL)
> >+		tdx->tdvmcall.rcx = vcpu->arch.regs[VCPU_REGS_RCX];
> 
> kvm_rcx_read()?
> 
> 		
> >+	else
> >+		tdx->tdvmcall.rcx = 0;
> 
> RCX on TDVMCALL exit is supposed to be consumed by TDX module. I don't get why
> caching it is necessary. Can tdx->tdvmcall be simply dropped?

Now it's not used. Will drop tdvmcall.

It was originally used to remember a original register mask of TDVMCALL, and
tdx_complete_vp_vmcall() used it as a valid value to copy back the output
values.  The current tdx_complete_vp_vmcall() uses kvm_rcx_read() because even
if the user space changes rcx, it doesn't harm to KVM.  KVM does what the user
space tells.
-- 
Isaku Yamahata <isaku.yamahata@intel.com>