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Subject: RE: [PATCH v10 36/38] x86/fred: Add fred_syscall_init()
Thread-Topic: [PATCH v10 36/38] x86/fred: Add fred_syscall_init()
Thread-Index: AQHZ5strXMSQ17oxYk2by6W1B7FxqLAh2QeAgAFNrPA=
Date:   Wed, 20 Sep 2023 04:33:38 +0000
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References: <20230914044805.301390-1-xin3.li@intel.com>
 <20230914044805.301390-37-xin3.li@intel.com> <87v8c6woqo.ffs@tglx>
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> > +static inline void fred_syscall_init(void) {
> > +	/*
> > +	 * Per FRED spec 5.0, FRED uses the ring 3 FRED entrypoint for SYSCAL=
L
> > +	 * 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);
>=20
> 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?

Thanks!
    Xin