linux-kernel - Re: [PATCHv2 1/4] x86/tdx: Introduce tdg_vm

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Date: Wed, 24 Apr 2024 10:53:00 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Cc: sathyanarayanan.kuppuswamy@...ux.intel.com, hpa@...or.com,
 seanjc@...gle.com, elena.reshetova@...el.com, rick.p.edgecombe@...el.com,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 1/4] x86/tdx: Introduce tdg_vm_wr()

So, the text here should help me understand what is going on.  What it
tries to tell me should not be a literal rephrasing of the contents of
the diff.

This patch literally introduces a function called tdg_vm_wr().  The
subject adds precisely zero to what I can get from reading the diff.

How about:

	x86/tdx: Factor out TD metadata write tdcall

On 3/25/24 03:46, Kirill A. Shutemov wrote:
> Add a helper function to write to a TD-scope metadata field and use it
> to set NOTIFY_ENABLES.
> 
> The helper function will be paired with tdg_vm_rd() and will be used to
> manipulate other metadata fields, not just NOTIFY_ENABLES.

Ideally this would at least tell me what problem this is solving:

	The TDG_VM_WR seamcall is used to ask the TDX module to change
	some TD-specific VM configuration.  There is currently only one
	user in the kernel of this seamcall.  More will be added
	shortly.

.. and then the solution:

	Refactor to make way for more users of TDG_VM_WR who will need
	to modify other TD configuration values.

> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 59776ce1c1d7..4fb36e5c4e80 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -77,6 +77,20 @@ static inline void tdcall(u64 fn, struct tdx_module_args *args)
>  		panic("TDCALL %lld failed (Buggy TDX module!)\n", fn);
>  }
>  
> +static inline u64 tdg_vm_wr(u64 field, u64 value, u64 mask)
> +{
> +	struct tdx_module_args args = {
> +		.rdx = field,
> +		.r8 = value,
> +		.r9 = mask,
> +	};
> +
> +	tdcall(TDG_VM_WR, &args);
> +
> +	/* Old value */
> +	return args.r8;
> +}
> +
>  /**
>   * tdx_mcall_get_report0() - Wrapper to get TDREPORT0 (a.k.a. TDREPORT
>   *                           subtype 0) using TDG.MR.REPORT TDCALL.
> @@ -902,10 +916,6 @@ static void tdx_kexec_unshare_mem(void)
>  
>  void __init tdx_early_init(void)
>  {
> -	struct tdx_module_args args = {
> -		.rdx = TDCS_NOTIFY_ENABLES,
> -		.r9 = -1ULL,
> -	};
>  	u64 cc_mask;
>  	u32 eax, sig[3];
>  
> @@ -924,7 +934,7 @@ void __init tdx_early_init(void)
>  	cc_set_mask(cc_mask);
>  
>  	/* Kernel does not use NOTIFY_ENABLES and does not need random #VEs */
> -	tdcall(TDG_VM_WR, &args);
> +	tdg_vm_wr(TDCS_NOTIFY_ENABLES, 0, -1ULL);
>  
>  	/*
>  	 * All bits above GPA width are reserved and kernel treats shared bit