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Subject: RE: [PATCH v10 36/38] x86/fred: Add fred_syscall_init()
In-Reply-To: <SA1PR11MB6734C02FFB973B2074EC6CC8A8F9A@SA1PR11MB6734.namprd11.prod.outlook.com>
References: <20230914044805.301390-1-xin3.li@intel.com>
 <20230914044805.301390-37-xin3.li@intel.com> <87v8c6woqo.ffs@tglx>
 <SA1PR11MB6734C02FFB973B2074EC6CC8A8F9A@SA1PR11MB6734.namprd11.prod.outlook.com>
Date:   Wed, 20 Sep 2023 10:18:14 +0200
Message-ID: <87h6npuuk9.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, Sep 20 2023 at 04:33, Li, Xin3 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().
>
> What I heard is that AMD is going to support FRED.
>
> Both LSTAR and CSTAR have no function when FRED is enabled, so maybe
> just do NOT write to them?

Right. If AMD needs to clear it then it's trivial enough to add a
wrmsrl_cstar(0) to it.