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Date:   Wed, 20 Sep 2023 19:24:43 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Thomas Gleixner <tglx@linutronix.de>,
        "Li, Xin3" <xin3.li@intel.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
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        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
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Subject: RE: [PATCH v10 36/38] x86/fred: Add fred_syscall_init()
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On September 20, 2023 1:18:14 AM PDT, Thomas Gleixner <tglx@linutronix=2Ede=
> wrote:
>On Wed, Sep 20 2023 at 04:33, Li, Xin3 wrote:
>>> > +static inline void fred_syscall_init(void) {
>>> > +	/*
>>> > +	 * Per FRED spec 5=2E0, FRED uses the ring 3 FRED entrypoint for S=
YSCALL
>>> > +	 * 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=2E
>>> > +	 *
>>> > +	 * Note, both sysexit and sysret cause #UD when FRED is enabled=2E
>>> > +	 */
>>> > +	wrmsrl(MSR_LSTAR, 0ULL);
>>> > +	wrmsrl_cstar(0ULL);
>>>=20
>>> That write is pointless=2E See the comment in wrmsrl_cstar()=2E
>>
>> What I heard is that AMD is going to support FRED=2E
>>
>> Both LSTAR and CSTAR have no function when FRED is enabled, so maybe
>> just do NOT write to them?
>
>Right=2E If AMD needs to clear it then it's trivial enough to add a
>wrmsrl_cstar(0) to it=2E

Just to clarify: the only reason I added the writes here was to possibly m=
ake bugs easier to track down=2E There is indeed no functional reason=2E