linux-kernel - Re: [RFC v2-fix 1/1] x86/boot: Add a trampoline for APs booting in 64-bit mode

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Date:   Wed, 19 May 2021 17:18:52 -0700
From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Kai Huang <kai.huang@...el.com>
Subject: Re: [RFC v2-fix 1/1] x86/boot: Add a trampoline for APs booting in
 64-bit mode

Hi Dan,

On 5/17/21 9:08 PM, Dan Williams wrote:
>> SYM_DATA_START_LOCAL(tr_idt)
>>           .short  0
>>           .quad   0
>> SYM_DATA_END(tr_idt)
> This format implies that tr_idt is reserving space for 2 distinct data
> structure attributes of those sizes, can you just put those names here
> as comments? Otherwise the .fill format is more compact.

Initially its 6 bytes (2 bytes for IDT limit, 4 bytes for 32 bit linear
start address). This patch extends it by another 4 bytes for supporting
64 bit mode.

2 bytes IDT limit (.short)
8 bytes for 64 bit IDT start address (.quad)

This info is included in commit log. But I will add comment here as you
have mentioned.

Will following comment log do ?

/* Use 10 bytes for IDT (in 64 bit mode), 8 bytes for IDT start address
    2 bytes for IDT limit size */

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer