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

[Huang, Kai]

On Thu, 2023-07-13 at 11:01 +0200, Peter Zijlstra wrote:
> 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.

This isn't true for the case that VP.ENTER returns due to a TDVMCALL.  In that
case RCX contains the bitmap of shared registers, and RBX/RDX/RDI/RSI/R8-R15
contains guest value if the corresponding bit is set in RCX (RBP will be
excluded by updating the spec I assume).

Or are you suggesting we need to decode RAX to decide whether the VP.ENTER
return is due to TDVMCALL vs other reasons, and act differently?