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Date:   Thu, 13 Jul 2023 11:01:10 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Huang, Kai" <kai.huang@intel.com>
Cc:     "Hansen, Dave" <dave.hansen@intel.com>,
        "Christopherson,, Sean" <seanjc@google.com>,
        "bp@alien8.de" <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "Yamahata, Isaku" <isaku.yamahata@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "sathyanarayanan.kuppuswamy@linux.intel.com" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Subject: Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more
 TDCALL/SEAMCALL leafs
Message-ID: <20230713090110.GC3138667@hirez.programming.kicks-ass.net>
References: <cover.1689151537.git.kai.huang@intel.com>
 <ecfd84af9186aa5368acb40a2740afbf1d0d1b5d.1689151537.git.kai.huang@intel.com>
 <20230712171133.GB3115257@hirez.programming.kicks-ass.net>
 <a36d1f0068154a9acd3fdbed2586dc5b2476e140.camel@intel.com>
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On Thu, Jul 13, 2023 at 08:09:33AM +0000, Huang, Kai wrote:
> On Wed, 2023-07-12 at 19:11 +0200, Peter Zijlstra wrote:
> > On Wed, Jul 12, 2023 at 08:55:21PM +1200, Kai Huang wrote:
> > > @@ -65,6 +104,37 @@
> > >  	.endif
> > >  
> > >  	.if \ret
> > > +	.if \saved
> > > +	/*
> > > +	 * Restore the structure from stack to saved the output registers
> > > +	 *
> > > +	 * In case of VP.ENTER returns due to TDVMCALL, all registers are
> > > +	 * valid thus no register can be used as spare to restore the
> > > +	 * structure from the stack (see "TDH.VP.ENTER Output Operands
> > > +	 * Definition on TDCALL(TDG.VP.VMCALL) Following a TD Entry").
> > > +	 * For this case, need to make one register as spare by saving it
> > > +	 * to the stack and then manually load the structure pointer to
> > > +	 * the spare register.
> > > +	 *
> > > +	 * Note for other TDCALLs/SEAMCALLs there are spare registers
> > > +	 * thus no need for such hack but just use this for all for now.
> > > +	 */
> > > +	pushq	%rax		/* save the TDCALL/SEAMCALL return code */
> > > +	movq	8(%rsp), %rax	/* restore the structure pointer */
> > > +	movq	%rsi, TDX_MODULE_rsi(%rax)	/* save %rsi */
> > > +	movq	%rax, %rsi	/* use %rsi as structure pointer */
> > > +	popq	%rax		/* restore the return code */
> > > +	popq	%rsi		/* pop the structure pointer */
> > 
> > Urgghh... At least for the \host case you can simply pop %rsi, no?
> > VP.ENTER returns with 0 there IIRC.
> 
> No VP.ENTER doesn't return 0 for RAX.  Firstly, VP.ENTER can return for many

No, but it *does* return 0 for: RBX,RSI,RDI,R10-R15.

So for \host you can simply do:

	pop	%rsi
	mov	$0, TDX_MODULE_rsi(%rsi)

and call it a day.