linux-kernel - Re: [PATCH v19 078/130] KVM: TDX: Implement TDX vcpu enter/exit path

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Date: Thu, 4 Apr 2024 15:45:35 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, Tina Zhang <tina.zhang@...el.com>, 
	Hang Yuan <hang.yuan@...el.com>, Bo Chen <chen.bo@...el.com>, 
	"sagis@...gle.com" <sagis@...gle.com>, "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, 
	Erdem Aktas <erdemaktas@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v19 078/130] KVM: TDX: Implement TDX vcpu enter/exit path

On Thu, Apr 04, 2024, Kai Huang wrote:
> On Thu, 2024-04-04 at 16:22 +0300, Kirill A. Shutemov wrote:
> > On Mon, Feb 26, 2024 at 12:26:20AM -0800, isaku.yamahata@...el.com wrote:
> > > @@ -491,6 +494,87 @@ void tdx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> > >  	 */
> > >  }
> > >  
> > > +static noinstr void tdx_vcpu_enter_exit(struct vcpu_tdx *tdx)
> > > +{
> > 
> > ...
> > 
> > > +	tdx->exit_reason.full = __seamcall_saved_ret(TDH_VP_ENTER, &args);
> > 
> > Call to __seamcall_saved_ret() leaves noinstr section.
> > 
> > __seamcall_saved_ret() has to be moved:
> > 
> > diff --git a/arch/x86/virt/vmx/tdx/seamcall.S b/arch/x86/virt/vmx/tdx/seamcall.S
> > index e32cf82ed47e..6b434ab12db6 100644
> > --- a/arch/x86/virt/vmx/tdx/seamcall.S
> > +++ b/arch/x86/virt/vmx/tdx/seamcall.S
> > @@ -44,6 +44,8 @@ SYM_FUNC_START(__seamcall_ret)
> >  SYM_FUNC_END(__seamcall_ret)
> >  EXPORT_SYMBOL_GPL(__seamcall_ret);
> >  
> > +.section .noinstr.text, "ax"
> > +
> >  /*
> >   * __seamcall_saved_ret() - Host-side interface functions to SEAM software
> >   * (the P-SEAMLDR or the TDX module), with saving output registers to the
> 
> Alternatively, I think we can explicitly use instrumentation_begin()/end()
> around __seamcall_saved_ret() here.

No, that will just paper over the complaint.  Dang it, I was going to say that
I called out earlier that tdx_vcpu_enter_exit() doesn't need to be noinstr, but
it looks like my brain and fingers didn't connect.

So I'll say it now :-)

I don't think tdx_vcpu_enter_exit() needs to be noinstr, because the SEAMCALL is
functionally a VM-Exit, and so all host state is saved/restored "atomically"
across the SEAMCALL (some by hardware, some by software (TDX-module)).

The reason the VM-Enter flows for VMX and SVM need to be noinstr is they do things
like load the guest's CR2, and handle NMI VM-Exits with NMIs blocks.  None of
that applies to TDX.  Either that, or there are some massive bugs lurking due to
missing code.