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SubjectRe: [PATCH v3 05/12] x86/tdx: Pass TDCALL/SEAMCALL input/output registers via a structure
On Thu, Jul 27, 2023 at 10:54:28PM +0000, Huang, Kai wrote:
> On Thu, 2023-07-27 at 19:36 +0300, kirill.shutemov@linux.intel.com 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?

Yes.

> 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.

As a user, if I see a struct used for in and out, I would expect that all
fields have the same rules.

> 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.

Do you always read documentation? :P Maybe it is only me...

> 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
>
> :-)

Consider the case whe the patch was (wrongly) backported to use new call
that uses R10 as input.

I realize that all my objections are rather hand-wavy. I would like to
have in/out symmetry here. But I would not NAK patch over this.

--
Kiryl Shutsemau / Kirill A. Shutemov

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