linux-kernel - Re: [PATCHv6 02/30] x86/tdx: Provide common base for SEAMCALL and TDCALL C wrappers

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Date:   Thu, 17 Mar 2022 00:33:58 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCHv6 02/30] x86/tdx: Provide common base for SEAMCALL and
 TDCALL C wrappers

Kirrill,

On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:
> Define an assembly macro that can be used to implement C wrapper for
> both TDCALL and SEAMCALL.
>

> TDCALL wrapper will be implemented using the macro later in the series.
> SEAMCALL wrapper is out-of-scope for the series and will be implemented
> as part of TDX host enabling.

This paragraph makes no sense in the changelog once it's merged. That's
a reviewer/maintainer info which wants to be below the ---

> +/*
> + * TDX_MODULE_CALL - common helper macro for both
> + *                 TDCALL and SEAMCALL instructions.
> + *
> + * TDCALL   - used by TDX guests to make requests to the
> + *            TDX module and hypercalls to the VMM.
> + * SEAMCALL - used by TDX hosts to make requests to the
> + *            TDX module.
> + *
> + * Both instruction are supported in Binutils >= 2.36.
> + */
> +#define tdcall		.byte 0x66,0x0f,0x01,0xcc
> +#define seamcall	.byte 0x66,0x0f,0x01,0xcf

Bah. I really hate this #define glue. The comment documents
TDX_MODULE_CALL. The defines for the instructions are a seperate
problem. So please move them above the whole thing with their own
comment.

> +.macro TDX_MODULE_CALL host:req
> +	/*
> +	 * R12 will be used as temporary storage for struct tdx_module_output
> +	 * pointer. Since R12-R15 registers are not used by TDCALL/SEAMCALL
> +	 * services supported by this function, it can be reused.
> +	 */
> +
> +	/* Callee saved, so preserve it */
> +	push %r12
> +
> +	/*
> +	 * Push output pointer to stack.
> +	 * After the operation, it will be fetched into R12 register.
> +	 */
> +	push %r9
> +
> +	/* Mangle function call ABI into TDCALL/SEAMCALL ABI: */
> +	/* Move Leaf ID to RAX */
> +	mov %rdi, %rax
> +	/* Move input 4 to R9 */
> +	mov %r8,  %r9
> +	/* Move input 3 to R8 */
> +	mov %rcx, %r8
> +	/* Move input 1 to RCX */
> +	mov %rsi, %rcx
> +	/* Leave input param 2 in RDX */
> +
> +	.if \host
> +	seamcall
> +	/*
> +	 * SEAMCALL instruction is essentially a VMExit from VMX root
> +	 * mode to SEAM VMX root mode.  VMfailInvalid (CF=1) indicates
> +	 * that the targeted SEAM firmware is not loaded or disabled,
> +	 * or P-SEAMLDR is busy with another SEAMCALL.  %rax is not
> +	 * changed in this case.
> +	 *
> +	 * Set %rax to TDX_SEAMCALL_VMFAILINVALID for VMfailInvalid.
> +	 * This value will never be used as actual SEAMCALL error code as
> +	 * it is from the Reserved status code class.
> +	 */
> +	jnc .Lno_vmfailinvalid
> +	mov $TDX_SEAMCALL_VMFAILINVALID, %rax
> +.Lno_vmfailinvalid:

Please add a new line between the label and the .else for readability sake.

> +	.else
> +	tdcall
> +	.endif
> +
> +	/*
> +	 * Fetch output pointer from stack to R12 (It is used
> +	 * as temporary storage)
> +	 */
> +	pop %r12
> +
> +	/*
> +	 * Since this function can be initiated without an output pointer,
> +	 * check if caller provided an output struct before storing output
> +	 * registers.

The function is a macro. It's not initiated, it's invoked. It always has
an output pointer in R12, but that can be NULL.

With those nitpicks addressed:

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>