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From:   Thomas Gleixner <tglx@linutronix.de>
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Subject: Re: [PATCH v10 36/38] x86/fred: Add fred_syscall_init()
In-Reply-To: <20230914044805.301390-37-xin3.li@intel.com>
References: <20230914044805.301390-1-xin3.li@intel.com>
 <20230914044805.301390-37-xin3.li@intel.com>
Date:   Tue, 19 Sep 2023 10:28:47 +0200
Message-ID: <87v8c6woqo.ffs@tglx>
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On Wed, Sep 13 2023 at 21:48, Xin Li wrote:
> +static inline void fred_syscall_init(void)
> +{
> +	/*
> +	 * Per FRED spec 5.0, FRED uses the ring 3 FRED entrypoint for SYSCALL
> +	 * and SYSENTER, and ERETU is the only legit instruction to return to
> +	 * ring 3, as a result there is _no_ need to setup the SYSCALL and
> +	 * SYSENTER MSRs.
> +	 *
> +	 * Note, both sysexit and sysret cause #UD when FRED is enabled.
> +	 */
> +	wrmsrl(MSR_LSTAR, 0ULL);
> +	wrmsrl_cstar(0ULL);

That write is pointless. See the comment in wrmsrl_cstar().

Thanks,

        tglx