linux-kernel - Re: [PATCH v2 0/6] Enable LKGS instruction

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Date:   Mon, 10 Oct 2022 17:59:34 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Xin Li <xin3.li@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, peterz@...radead.org
Subject: Re: [PATCH v2 0/6] Enable LKGS instruction

On 10/10/22 12:01, Xin Li wrote:
> LKGS instruction is introduced with Intel FRED (flexible return and event
> delivery) specification https://cdrdv2.intel.com/v1/dl/getContent/678938.
> 
> LKGS is independent of FRED, so we enable it as a standalone CPU feature.
> 
> LKGS behaves like the MOV to GS instruction except that it loads the base
> address into the IA32_KERNEL_GS_BASE MSR instead of the GS segment’s
> descriptor cache, which is exactly what Linux kernel does to load user level
> GS base.  Thus, with LKGS, there is no need to SWAPGS away from the kernel
> GS base.
> 
> Changes since V1:
> * place fixup code into code section "__ex_table" instead of the obsoleted
>    "fixup" section.
> 

Correction: __ex_table is NOT a code section (scared me there for a 
second...). With the new fixup handling code EX_TYPE_ZERO_REG takes care 
of all the work, and there simply is no need for any fixup code at all 
(the exception fixup is fully data-driven.)

So I would say "use EX_TYPE_ZERO_REG instead of fixup code in the 
obsolete .fixup code section."

	-hpa