Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
IronPort-SDR: iFNby29po1uDWdFmL5AOwkFICJfoq/W/fdd2ALkX//pWWc3TW4pMCPq08/nnv/xy1lf3vo0yIH
 wxb2mq985A+g==
IronPort-SDR: QlGRTqEN4d8qtBxrb2skgkdPLBIZM2kFqgtNC1yKwoS0Zp6q+/PfjONCBSwsAQL+HfayEcCDz5
 SNVLTsJ6l8EQ==
Date:   Tue, 29 Sep 2020 01:07:40 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>, "H.J. Lu" <hjl.tools@gmail.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        linux-sgx@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
        Cedric Xing <cedric.xing@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        asapek@google.com, Borislav Petkov <bp@alien8.de>,
        chenalexchen@google.com, Conrad Parker <conradparker@google.com>,
        cyhanish@google.com, "Huang, Haitao" <haitao.huang@intel.com>,
        Josh Triplett <josh@joshtriplett.org>,
        "Huang, Kai" <kai.huang@intel.com>,
        "Svahn, Kai" <kai.svahn@intel.com>, Keith Moyer <kmoy@google.com>,
        Christian Ludloff <ludloff@google.com>,
        Neil Horman <nhorman@redhat.com>,
        Nathaniel McCallum <npmccallum@redhat.com>,
        Patrick Uiterwijk <puiterwijk@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Thomas Gleixner <tglx@linutronix.de>, yaozhangx@google.com,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
Message-ID: <20200928220740.GG2705@linux.intel.com>
References: <20200915112842.897265-1-jarkko.sakkinen@linux.intel.com>
 <20200915112842.897265-22-jarkko.sakkinen@linux.intel.com>
 <721ca14e-21df-3df1-7bef-0b00d0ff90c3@citrix.com>
 <20200928005842.GC6704@linux.intel.com>
 <85bc15d5-93cd-e332-ae9a-1e1e66e1181d@citrix.com>
 <CAMe9rOpzXW0cSD=9E7drGEHH=pcm_NqvPiaR0pBJzYLeAt0_3g@mail.gmail.com>
 <CALCETrU4Rhc0fwzzKLSUgan2YmSovxVFYOZEmFnBHC4DbZ5RfQ@mail.gmail.com>
 <0edfbf96-32db-3565-0d07-7d4a4118dbe6@intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <0edfbf96-32db-3565-0d07-7d4a4118dbe6@intel.com>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
Precedence: bulk

On Mon, Sep 28, 2020 at 11:17:42AM -0700, Dave Hansen wrote:
> On 9/28/20 11:12 AM, Andy Lutomirski wrote:
> >> endbr64
> >> /* Check if shadow stack is in use.  NB: R11 is the only usable
> >>    scratch register for function calls.  */
> >> xorl %r11d, %r11d
> >> rdsspq %r11
> >> testq %r11, %r11
> >> jnz 3f
> >> call 2f
> >> 1:
> >> pause
> >> lfence
> >> jmp 1b
> >> 2:
> >> mov %rax, (%rsp)
> >> ret
> >> 3:
> >> /* Shadow stack is in use.  Make the indirect call.  */
> >> call *%rax
> >> ret
> > What do we expect user programs to do on CET systems?  It would be
> > nice if we could instead ALTERNATIVE this out if X86_FEATURE_SHSTK.
> 
> Shouldn't we just be able to use X86_FEATURE_RETPOLINE?
> 
> We probably need a mechanism to force X86_FEATURE_SHSTK and
> X86_FEATURE_RETPOLINE to be mutually exclusive if we don't have one already.

First of all: lets go with boot time patching instead of dynamic
detection. It's both easier to implement and by all other merits makes a
lot more sense. It was just a thing that I've not used before.

That sorted out, does it matter which direction I look it at? I could
use either feature flag as basis (and I do not have a personal
preference here).

/Jarkko