linux-kernel - Re: [PATCH 03/20] x86/tdx: Convert port I/O handling to use new TDVMCALL macros

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Message-ID: <611a387b-ba7e-46d7-b6bf-84dc6c037d33@redhat.com>
Date: Fri, 17 May 2024 19:37:38 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Dave Hansen <dave.hansen@...el.com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Sean Christopherson <seanjc@...gle.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, "K. Y. Srinivasan" <kys@...rosoft.com>,
 Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>
Cc: linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-hyperv@...r.kernel.org
Subject: Re: [PATCH 03/20] x86/tdx: Convert port I/O handling to use new
 TDVMCALL macros

On 5/17/24 17:28, Dave Hansen wrote:
> On 5/17/24 07:19, Kirill A. Shutemov wrote:
>>   static inline void tdx_io_out(int size, u16 port, u32 value)
>>   {
>> -	struct tdx_module_args args = {
>> -		.r10 = TDX_HYPERCALL_STANDARD,
>> -		.r11 = hcall_func(EXIT_REASON_IO_INSTRUCTION),
>> -		.r12 = size,
>> -		.r13 = 1,
>> -		.r14 = port,
>> -		.r15 = value,
>> -	};
>> -
>> -	__tdx_hypercall(&args);
>> +	TDVMCALL_0(hcall_func(EXIT_REASON_IO_INSTRUCTION),
>> +		   size, TDX_PORT_WRITE, port, value);
>>   }
> 
> I actually really like the self-documenting nature of the structures.  I
> don't think it's a win if this is where the lines-of-code savings comes
> from.
> 

It's just a tradeoff.  For example someone could well have written

#define TDVMCALL_0(reason, a1, a2, a3, a4) \
   do { \
	struct tdx_module_args args = {
		.r10 = TDX_HYPERCALL_STANDARD,
		.r11 = reason,
		.r12 = a1,
		.r13 = a2,
		.r14 = a3,
		.r15 = a4,
	__tdx_hypercall(&args);
   } while(0)

even with the current __tdx_hypercall() implementation.

I agree that TDVMCALL_x is somewhat less legible; on the other hand it 
highlights that these TDVMCALLs all have a common convention for passing 
parameters / retrieving results, and reduces the potential for silly typos.

This is also why I asked about the different approaches for TDCALL vs. 
TDVMCALL.  Given that there are only a handful of appearances for 
tdvmcall_trampoline, maybe the best of both worlds is just to inline the 
whole thing?  This way the code in the macros matches the parameter 
passing convention of the GHCI.

Paolo