linux-kernel - Re: [RFC][PATCH 9/9] arch/idle: Change arch_cpu

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Message-ID: <20220520022052.mkrc4v4evtp74bxe@black.fi.intel.com>
Date:   Fri, 20 May 2022 05:20:52 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     frederic@...nel.org, paulmck@...nel.org, rjw@...ysocki.net,
        x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...nel.org
Subject: Re: [RFC][PATCH 9/9] arch/idle: Change arch_cpu_idle() IRQ behaviour

On Fri, May 20, 2022 at 12:03:49AM +0200, Peter Zijlstra wrote:
> 
> On Thu, May 19, 2022 at 11:27:59PM +0200, Peter Zijlstra wrote:
> > --- a/arch/x86/coco/tdx/tdx.c
> > +++ b/arch/x86/coco/tdx/tdx.c
> > @@ -178,6 +178,9 @@ void __cpuidle tdx_safe_halt(void)
> >  	 */
> >  	if (__halt(irq_disabled, do_sti))
> >  		WARN_ONCE(1, "HLT instruction emulation failed\n");
> > +
> > +	/* XXX I can't make sense of what @do_sti actually does */
> > +	raw_local_irq_disable();
> >  }
> >  
> 
> Kirill, Dave says I should prod you :-)

It calls STI just before doing TDCALL that requests HLT.
See comment above $TDX_HCALL_ISSUE_STI usage in __tdx_hypercall()[1].

__halt(do_sti == true) matches native_safe_halt() semantics (or suppose
to) and __halt(do_sti == false) corresponds to native_halt().

For context, see Section 3.8 in GHCI[2]

[1] https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/arch/x86/coco/tdx/tdcall.S?h=x86/tdx#n151
[2] https://www.intel.com/content/dam/develop/external/us/en/documents/intel-tdx-guest-hypervisor-communication-interface-1.0-344426-002.pdf

-- 
 Kirill A. Shutemov