linux-kernel - Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more TDCALL/SEAMCALL leafs

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Date:   Wed, 12 Jul 2023 19:11:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kai Huang <kai.huang@...el.com>
Cc:     kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
        dave.hansen@...el.com, tglx@...utronix.de, bp@...en8.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org, seanjc@...gle.com,
        pbonzini@...hat.com, kvm@...r.kernel.org, isaku.yamahata@...el.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more
 TDCALL/SEAMCALL leafs

On Wed, Jul 12, 2023 at 08:55:21PM +1200, Kai Huang wrote:
> @@ -65,6 +104,37 @@
>  	.endif
>  
>  	.if \ret
> +	.if \saved
> +	/*
> +	 * Restore the structure from stack to saved the output registers
> +	 *
> +	 * In case of VP.ENTER returns due to TDVMCALL, all registers are
> +	 * valid thus no register can be used as spare to restore the
> +	 * structure from the stack (see "TDH.VP.ENTER Output Operands
> +	 * Definition on TDCALL(TDG.VP.VMCALL) Following a TD Entry").
> +	 * For this case, need to make one register as spare by saving it
> +	 * to the stack and then manually load the structure pointer to
> +	 * the spare register.
> +	 *
> +	 * Note for other TDCALLs/SEAMCALLs there are spare registers
> +	 * thus no need for such hack but just use this for all for now.
> +	 */
> +	pushq	%rax		/* save the TDCALL/SEAMCALL return code */
> +	movq	8(%rsp), %rax	/* restore the structure pointer */
> +	movq	%rsi, TDX_MODULE_rsi(%rax)	/* save %rsi */
> +	movq	%rax, %rsi	/* use %rsi as structure pointer */
> +	popq	%rax		/* restore the return code */
> +	popq	%rsi		/* pop the structure pointer */

Urgghh... At least for the \host case you can simply pop %rsi, no?
VP.ENTER returns with 0 there IIRC.

	/* xor-swap (%rsp) and %rax */
	xorq	(%rsp), %rax
	xorq	%rax, (%rsp)
	xorq	(%rsp), %rax

	movq	%rsi, TDX_MODULE_rsi(%rax);
	movq	%rax, %rsi

	popq	%rax

Isn't much better is it :/ Too bad xchg implies lock prefix.

> +
> +	/* Copy additional output regs to the structure  */
> +	movq %r12, TDX_MODULE_r12(%rsi)
> +	movq %r13, TDX_MODULE_r13(%rsi)
> +	movq %r14, TDX_MODULE_r14(%rsi)
> +	movq %r15, TDX_MODULE_r15(%rsi)
> +	movq %rbx, TDX_MODULE_rbx(%rsi)
> +	movq %rdi, TDX_MODULE_rdi(%rsi)
> +	.endif	/* \saved */
> +
>  	/* Copy output regs to the structure */
>  	movq %rcx, TDX_MODULE_rcx(%rsi)
>  	movq %rdx, TDX_MODULE_rdx(%rsi)