linux-kernel - Re: [PATCHv2.1 05/29] x86/tdx: Add HLT support for TDX guests

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Date:   Tue, 01 Feb 2022 22:21:58 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        bp@...en8.de
Cc:     aarcange@...hat.com, ak@...ux.intel.com, dan.j.williams@...el.com,
        dave.hansen@...el.com, david@...hat.com, hpa@...or.com,
        jgross@...e.com, jmattson@...gle.com, joro@...tes.org,
        jpoimboe@...hat.com, kirill.shutemov@...ux.intel.com,
        knsathya@...nel.org, linux-kernel@...r.kernel.org, luto@...nel.org,
        mingo@...hat.com, pbonzini@...hat.com, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, sdeep@...are.com,
        seanjc@...gle.com, tony.luck@...el.com, vkuznets@...hat.com,
        wanpengli@...cent.com, x86@...nel.org
Subject: Re: [PATCHv2.1 05/29] x86/tdx: Add HLT support for TDX guests

On Sun, Jan 30 2022 at 01:30, Kirill A. Shutemov wrote:
> +/*
> + * Used in __tdx_hypercall() to determine whether to enable interrupts
> + * before issuing TDCALL for the EXIT_REASON_HLT case.
> + */
> +#define ENABLE_IRQS_BEFORE_HLT 0x01
> +
>  /*
>   * __tdx_module_call()  - Used by TDX guests to request services from
>   * the TDX module (does not include VMM services).
> @@ -230,6 +237,30 @@ SYM_FUNC_START(__tdx_hypercall)
>  
>  	movl $TDVMCALL_EXPOSE_REGS_MASK, %ecx
>  
> +	/*
> +	 * For the idle loop STI needs to be called directly before
> +	 * the TDCALL that enters idle (EXIT_REASON_HLT case). STI
> +	 * instruction enables interrupts only one instruction later.
> +	 * If there is a window between STI and the instruction that
> +	 * emulates the HALT state, there is a chance for interrupts to
> +	 * happen in this window, which can delay the HLT operation
> +	 * indefinitely. Since this is the not the desired result,
> +	 * conditionally call STI before TDCALL.
> +	 *
> +	 * Since STI instruction is only required for the idle case
> +	 * (a special case of EXIT_REASON_HLT), use the r15 register
> +	 * value to identify it. Since the R15 register is not used
> +	 * by the VMM as per EXIT_REASON_HLT ABI, re-use it in
> +	 * software to identify the STI case.
> +	 */
> +	cmpl $EXIT_REASON_HLT, %r11d
> +	jne .Lskip_sti
> +	cmpl $ENABLE_IRQS_BEFORE_HLT, %r15d
> +	jne .Lskip_sti
> +	/* Set R15 register to 0, it is unused in EXIT_REASON_HLT case */
> +	xor %r15, %r15
> +	sti
> +.Lskip_sti:
>  	tdcall

This really can be simplified:

        cmpl	$EXIT_REASON_SAFE_HLT, %r11d
        jne	.Lnohalt
        movl	$EXIT_REASON_HLT, %r11d
        sti
.Lnohalt:
	tdcall

and the below becomes:

static bool tdx_halt(void)
{
	return !!__tdx_hypercall(EXIT_REASON_HLT, !!irqs_disabled(), 0, 0, 0, NULL);
}

void __cpuidle tdx_safe_halt(void)
{
        if (__tdx_hypercall(EXIT_REASON_SAFE_HLT, 0, 0, 0, 0, NULL)
        	WARN_ONCE(1, "HLT instruction emulation failed\n");
}

Hmm?

Thanks,

        tglx