RE: [PATCH v10 36/38] x86/fred: Add fred_syscall_init()

[H. Peter Anvin]

On September 20, 2023 1:18:14 AM PDT, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>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.

Just to clarify: the only reason I added the writes here was to possibly make bugs easier to track down. There is indeed no functional reason.