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

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Date:   Wed, 2 Feb 2022 15:48:30 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     bp@...en8.de, 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, 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 Tue, Feb 01, 2022 at 10:21:58PM +0100, Thomas Gleixner wrote:
> 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?

EXIT_REASON_* are architectural, see SDM vol 3D, appendix C. There's no
EXIT_REASON_SAFE_HLT.

Do you want to define a synthetic one? Like

#define EXIT_REASON_SAFE_HLT	0x10000

?

Looks dubious to me, I donno. I worry about possible conflicts with the
spec in the future.

-- 
 Kirill A. Shutemov