linux-kernel - Re: [PATCHv2 13/13] x86/acpi: Add support for CPU offlining for ACPI MADT wakeup method

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Date:   Wed, 25 Oct 2023 03:50:53 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>
CC:     "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Hunter, Adrian" <adrian.hunter@...el.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ashish.kalra@....com" <ashish.kalra@....com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "bhe@...hat.com" <bhe@...hat.com>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>
Subject: Re: [PATCHv2 13/13] x86/acpi: Add support for CPU offlining for ACPI
 MADT wakeup method


> > +	.text
> > +	.align PAGE_SIZE
> > +SYM_FUNC_START(asm_acpi_mp_play_dead)
> > +	/* Load address of reset vector into RCX to jump when kernel is ready */
> > +	movq	acpi_mp_reset_vector_paddr(%rip), %rcx
> > +
> > +	/* Turn off global entries. Following CR3 write will flush them. */
> > +	movq	%cr4, %rdx
> > +	andq	$~(X86_CR4_PGE), %rdx
> > +	movq	%rdx, %cr4
> > +
> > +	/* Switch to identity mapping */
> > +	movq	acpi_mp_pgd(%rip), %rax
> > +	movq	%rax, %cr3
> 
> Do we need to switch back to kernel direct-map page table after CPU is wake up
> again?  We do support normal CPU offline/online, but not limited to kexec,
> right?

Please ignore this.  I found if I am reading right even for TDX guest the new
online cpu will start with trampoline_start64 assembly, so it will load kernel
page table anyway.  Sorry for the noise.

[...]


> > +	for (int i = 0; i < nr_pfn_mapped; i++) {
> > +		unsigned long mstart, mend;
> > +		mstart = pfn_mapped[i].start << PAGE_SHIFT;
> > +		mend   = pfn_mapped[i].end << PAGE_SHIFT;
> > +		if (kernel_ident_mapping_init(&info, pgd, mstart, mend))
> > +			return -ENOMEM;
> > +	}
> 
> This is for kexec() IIUC.  Add a comment?
> 
> If we consider normal CPU offline/online case, then I don't think we need the
> identity mapping for all memory?
> 

Also this one. :-)

>