linux-kernel - Re: [PATCH v4 3/8] x86/virt/tdx: Prepare to support reading other global metadata fields

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Message-ID: <1b14e28b-972e-4277-898f-8e2dcb77e144@intel.com>
Date: Fri, 27 Sep 2024 11:05:48 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: "Hansen, Dave" <dave.hansen@...el.com>, "kirill.shutemov@...ux.intel.com"
	<kirill.shutemov@...ux.intel.com>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"bp@...en8.de" <bp@...en8.de>, "peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>, "hpa@...or.com" <hpa@...or.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>
CC: "x86@...nel.org" <x86@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "Hunter, Adrian" <adrian.hunter@...el.com>,
	"nik.borisov@...e.com" <nik.borisov@...e.com>
Subject: Re: [PATCH v4 3/8] x86/virt/tdx: Prepare to support reading other
 global metadata fields



On 27/09/2024 10:26 am, Hansen, Dave wrote:
> On 9/26/24 15:22, Huang, Kai wrote:
>> But Dan commented using typeless 'void *' and 'size' is kinda a step
>> backwards and we should do something similar to build_mmio_read():
> 
> Well, void* is typeless, but at least it knows the size in this case.
> It's not completely aimless.  I was thinking of how things like
> get_user() work.

get_user(x,ptr) only works with simple types:

  * @ptr must have pointer-to-simple-variable type, and the result of
  * dereferencing @ptr must be assignable to @x without a cast.

The compiler knows the type of both @x and @(*ptr), so it knows 
type-safety and size to copy.

I think we can eliminate the __read_sys_metadata_field() by implementing 
it as a macro directly and get rid of 'void *' and 'size':

static int tdh_sys_rd(u64 field_id, u64 *val) {}

/* @_valptr must be pointer to u8/u16/u32/u64 */
#define read_sys_metadata_field(_field_id, _valptr)	\
({							\
	u64 ___tmp;					\
	int ___ret;					\
							\
	BUILD_BUG_ON(MD_FIELD_ELE_SIZE(_field_id) !=	\
		sizeof(*_valptr));			\
							\
	___ret = tdh_sys_rd(_field_id, &___tmp);	\
							\
	*_valptr = ___tmp;				\
	___ret;
})

It sets *_valptr unconditionally but we can also only do it when ___ret 
is 0.

The caller will need to do:

static int get_tdx_metadata_X_which_is_32bit(...)
{
	u32 metadata_X;
	int ret;

	ret = read_sys_metadata_field(MD_FIELD_ID_X, &metadata_X);

	return ret;
}

I haven't compiled and tested but it seems feasible.

Any comments?