linux-kernel - RE: [PATCH v10 36/38] x86/fred: Add fred_syscall

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Date:   Wed, 20 Sep 2023 19:24:43 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Li, Xin3" <xin3.li@...el.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Gross, Jurgen" <jgross@...e.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>
Subject: RE: [PATCH v10 36/38] x86/fred: Add fred_syscall_init()

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