Re: [PATCH v3 05/12] x86/tdx: Pass TDCALL/SEAMCALL input/output registers via a structure

[Huang, Kai]

On Thu, 2023-07-27 at 19:36 +0300, kirill.shutemov@xxxxxxxxxxxxxxx wrote:
> On Wed, Jul 26, 2023 at 11:25:07PM +1200, Kai Huang wrote:
> > diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
> > index 6bdf6e137953..a0e7fe81bf63 100644
> > --- a/arch/x86/virt/vmx/tdx/tdxcall.S
> > +++ b/arch/x86/virt/vmx/tdx/tdxcall.S
> > @@ -17,34 +17,33 @@
> >   *            TDX module and hypercalls to the VMM.
> >   * SEAMCALL - used by TDX hosts to make requests to the
> >   *            TDX module.
> > + *
> > + *-------------------------------------------------------------------------
> > + * TDCALL/SEAMCALL ABI:
> > + *-------------------------------------------------------------------------
> > + * Input Registers:
> > + *
> > + * RAX                 - TDCALL/SEAMCALL Leaf number.
> > + * RCX,RDX,R8-R9       - TDCALL/SEAMCALL Leaf specific input registers.
> > + *
> > + * Output Registers:
> > + *
> > + * RAX                 - TDCALL/SEAMCALL instruction error code.
> > + * RCX,RDX,R8-R11      - TDCALL/SEAMCALL Leaf specific output registers.
> > + *
> > + *-------------------------------------------------------------------------
> 
> So, you keep the existing asymetry in IN and OUT registers. R10 and R11
> are OUT-only registers. It can be confusing for user since it is the same
> structure now. I guess it changes in the following patches, but I would
> prefer to make them even here if there's no good reason not to.
> 


Do you mean you prefer to use R10/R11 as input too even in this patch?

I think _logically_ it should be part of the next patch, because w/o extending
TDX_MODULE_CALL to support additional TDCALLs/SEAMCALLs, we don't need R10/R11
as input.  This patch only changes to take a structure as function argument,
rather than taking individual registers as argument.

Also, we have a comment to say this around the structure too:

 /*
- * Used in __tdx_module_call() to gather the output registers' values of the
+ * Used in __tdcall*() to gather the input/output registers' values of the
  * TDCALL instruction when requesting services from the TDX module. This is a
  * software only structure and not part of the TDX module/VMM ABI
  */
-struct tdx_module_output {
+struct tdx_module_args {
+	/* input/output */
 	u64 rcx;
 	u64 rdx;
 	u64 r8;
 	u64 r9;
+	/* additional output */
 	u64 r10;
 	u64 r11;
 };

So to me there should be no confusion.

Even consider a theoretical case: someone wants to backport this patch to an old
kernel w/o further patches, then it makes little sense to do R10/R11 in
TDX_MODULE_CALL here in this patch

:-)