linux-kernel - Re: [PATCH v8 21/33] x86/fred: FRED initialization code

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Date:   Mon, 05 Jun 2023 14:15:25 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xin Li <xin3.li@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, peterz@...radead.org, andrew.cooper3@...rix.com,
        seanjc@...gle.com, pbonzini@...hat.com, ravi.v.shankar@...el.com,
        jiangshanlai@...il.com, shan.kang@...el.com
Subject: Re: [PATCH v8 21/33] x86/fred: FRED initialization code

On Mon, Apr 10 2023 at 01:14, Xin Li wrote:
> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
>
> The code to initialize FRED when it's available and _not_ disabled.
>
> cpu_init_fred_exceptions() is the core function to initialize FRED,
> which
>   1. Sets up FRED entrypoints for events happening in ring 0 and 3.
>   2. Sets up a default stack for event handling.
>   3. Sets up dedicated event stacks for DB/NMI/MC/DF, equivalent to
>      the IDT IST stacks.
>   4. Forces 32-bit system calls to use "int $0x80" only.
>   5. Enables FRED and invalidtes IDT.
>
> When the FRED is used, cpu_init_exception_handling() initializes FRED
> through calling cpu_init_fred_exceptions(), otherwise it sets up TSS
> IST and loads IDT.
>
> As FRED uses the ring 3 FRED entrypoint for SYSCALL and SYSENTER,
> it skips setting up SYSCALL/SYSENTER related MSRs, e.g., MSR_LSTAR.

So how is this supposed to work? FRED is enabled in Kconfig, the feature
is detected and FRED is initialized _before_ the rest of the required
changes is in place.

Documentation/process/* is not just there because people have nothing
better to do than writing pointless documents.

Thanks,

        tglx