Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more TDCALL/SEAMCALL leafs

[Huang, Kai]

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
reasons but not limited to TDVMCALL (e.g., due to interrupt), and for those
cases RAX contains valid non-zero value.  Secondly, even for TDVMCALL, the RAX
isn't 0:

Table 6.256 TDX 1.5 ABI spec:

RAX	SEAMCALL instruction return code
	The DETAILS_L2 field in bits 31:0 contain the VMCS exit reason, 
	indicating TDCALL (77).