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

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Date:   Wed, 02 Feb 2022 18:17:08 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
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 Wed, Feb 02 2022 at 15:48, Kirill A. Shutemov wrote:

> 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:
>> 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
> ?

That was my idea, yes.

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

The spec should have a reserved space for such things :)

But you might think about having a in/out struct similar to the module
call or just an array of u64.

and the signature would become:

__tdx_hypercall(u64 op, u64 flags, struct inout *args)
__tdx_hypercall(u64 op, u64 flags, u64 *args)

and have flag bits:

    HCALL_ISSUE_STI
    HCALL_HAS_OUTPUT

Hmm?

Thanks,

        tglx