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

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Date:   Thu, 3 Aug 2023 11:35:48 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "n.borisov.lkml@...il.com" <n.borisov.lkml@...il.com>
Subject: Re: [PATCH v3 05/12] x86/tdx: Pass TDCALL/SEAMCALL input/output
 registers via a structure

On Thu, 2023-08-03 at 13:58 +0300, kirill.shutemov@...ux.intel.com wrote:
> On Thu, Jul 27, 2023 at 10:54:28PM +0000, Huang, Kai wrote:
> > On Thu, 2023-07-27 at 19:36 +0300, kirill.shutemov@...ux.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.
> 

The only concern is I don't particularly like to add additional logic to this
patch.  Anyway not big deal to me.  I can do what you said if I don't see
Peter's (or other maintainers') comment on this.

Btw, should I say something like below in the changelog to justify this
additional logic:

	Also use R10/R11 as input registers too to make the input/output 
	registers symmetric, although currently no TDCALL/SEAMCALL use
	them as input registers.

?