linux-kernel - Re: [PATCH RFC 1/7] x86/virt/tdx: Add SEAMCALL wrapper to enter/exit TDX guest

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Message-ID: <91ebbd09-a02b-4287-a192-9259a6a78c4c@intel.com>
Date: Fri, 13 Dec 2024 08:44:16 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Adrian Hunter <adrian.hunter@...el.com>, pbonzini@...hat.com,
 seanjc@...gle.com, kvm@...r.kernel.org, dave.hansen@...ux.intel.com
Cc: rick.p.edgecombe@...el.com, kai.huang@...el.com,
 reinette.chatre@...el.com, xiaoyao.li@...el.com,
 tony.lindgren@...ux.intel.com, binbin.wu@...ux.intel.com,
 dmatlack@...gle.com, isaku.yamahata@...el.com, nik.borisov@...e.com,
 linux-kernel@...r.kernel.org, x86@...nel.org, yan.y.zhao@...el.com,
 chao.gao@...el.com, weijiang.yang@...el.com
Subject: Re: [PATCH RFC 1/7] x86/virt/tdx: Add SEAMCALL wrapper to enter/exit
 TDX guest

On 12/13/24 08:30, Adrian Hunter wrote:
> On 13/12/24 18:16, Dave Hansen wrote:
>> On 12/11/24 10:43, Adrian Hunter wrote:
>> ...
>>> -	size = tdvmcall_a0_read(vcpu);
>>> -	write = tdvmcall_a1_read(vcpu);
>>> -	port = tdvmcall_a2_read(vcpu);
>>> +	size  = tdx->vp_enter_out.io_size;
>>> +	write = tdx->vp_enter_out.io_direction == TDX_WRITE;
>>> +	port  = tdx->vp_enter_out.io_port;
>> ...> +	case TDVMCALL_IO:
>>> +		out->io_size		= args.r12;
>>> +		out->io_direction	= args.r13 ? TDX_WRITE : TDX_READ;
>>> +		out->io_port		= args.r14;
>>> +		out->io_value		= args.r15;
>>> +		break;
>>
>> I honestly don't understand the need for the abstracted structure to sit
>> in the middle. It doesn't get stored or serialized or anything, right?
>> So why have _another_ structure?
>>
>> Why can't this just be (for instance):
>>
>> 	size = tdx->foo.r12;
>>
>> ?
>>
>> Basically, you hand around the raw arguments until you need to use them.
> 
> That sounds like what we have at present?  That is:
> 
> u64 tdh_vp_enter(u64 tdvpr, struct tdx_module_args *args)
> {
> 	args->rcx = tdvpr;
> 
> 	return __seamcall_saved_ret(TDH_VP_ENTER, args);
> }
> 
> And then either add Rick's struct tdx_vp?  Like so:
> 
> u64 tdh_vp_enter(struct tdx_vp *vp, struct tdx_module_args *args)
> {
> 	args->rcx = tdx_tdvpr_pa(vp);
> 
> 	return __seamcall_saved_ret(TDH_VP_ENTER, args);
> }
> 
> Or leave it to the caller:
> 
> u64 tdh_vp_enter(struct tdx_module_args *args)
> {
> 	return __seamcall_saved_ret(TDH_VP_ENTER, args);
> }
> 
> Or forget the wrapper altogether, and let KVM call
> __seamcall_saved_ret() ?

Rick's version, please.

I don't want __seamcall_saved_ret() exported to modules. I want to at
least have a clean boundary beyond which __seamcall_saved_ret() is not
exposed.

My nit with the "u64 tdvpr" version was that there's zero type safety.

The tdvp-less tdh_vp_enter() is even *less* safe of a calling convention
and also requires that each caller do tdx_tdvpr_pa() or equivalent.

But I feel like I'm repeating myself a bit at this point.