linux-kernel - Re: [PATCH v5 5/5] x86/gsseg: use the LKGS instruction if available for load_gs

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Message-ID: <Y77H+rThnJixLEQC@zn.tnic>
Date:   Wed, 11 Jan 2023 15:30:18 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Xin Li <xin3.li@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, hpa@...or.com,
        peterz@...radead.org, brgerst@...il.com, chang.seok.bae@...el.com,
        jgross@...e.com
Subject: Re: [PATCH v5 5/5] x86/gsseg: use the LKGS instruction if available
 for load_gs_index()

On Mon, Nov 28, 2022 at 08:40:28AM -0800, Xin Li wrote:
> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
> 
> The LKGS instruction atomically loads a segment descriptor into the
> %gs descriptor registers, *except* that %gs.base is unchanged, and the
> base is instead loaded into MSR_IA32_KERNEL_GS_BASE, which is exactly
> what we want this function to do.
> 
> Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Brian Gerst <brgerst@...il.com>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> Signed-off-by: Xin Li <xin3.li@...el.com>

I'm reading this SOB chain as

hpa wrote it -> then it went to Peter -> then to Brian -> then to Juergen -> and
you're sending it.

I'm pretty sure that cannot be right.

> ---
> 
> Changes since v4:
> * Clear the LKGS feature from Xen PV guests (Juergen Gross).
> 
> Changes since v3:
> * We want less ASM not more, thus keep local_irq_save/restore() inside
>   native_load_gs_index() (Thomas Gleixner).
> * For paravirt enabled kernels, initialize pv_ops.cpu.load_gs_index to
>   native_lkgs (Thomas Gleixner).
> 
> Changes since v2:
> * Mark DI as input and output (+D) as in V1, since the exception handler
>   modifies it (Brian Gerst).
> 
> Changes since v1:
> * Use EX_TYPE_ZERO_REG instead of fixup code in the obsolete .fixup code
>   section (Peter Zijlstra).
> * Add a comment that states the LKGS_DI macro will be repalced with "lkgs %di"
>   once the binutils support the LKGS instruction (Peter Zijlstra).

I guess that explains what the SOB chain is supposed to mean - you've gotten
review feedback. But that doesn't need such a SOB chain. Sounds like you need to
refresh on

Documentation/process/submitting-patches.rst

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette