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Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Date:   Mon, 10 Jun 2019 15:59:29 -0700
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On 6/10/19 3:40 PM, Yu-cheng Yu wrote:
> Ok, we will go back to do_mmap() with MAP_PRIVATE, MAP_NORESERVE and
> VM_DONTDUMP.  The bitmap will cover only 48-bit address space.

Could you make sure to discuss the downsides of only doing a 48-bit
address space?

What are the reasons behind and implications of VM_DONTDUMP?

> We then create PR_MARK_CODE_AS_LEGACY.  The kernel will set the bitmap, but it
> is going to be slow.

Slow compared to what?  We're effectively adding one (quick) system call
to a path that, today, has at *least* half a dozen syscalls and probably
a bunch of page faults.  Heck, we can probably avoid the actual page
fault to populate the bitmap if we're careful.  That alone would put a
syscall on equal footing with any other approach.  If the bit setting
crossed a page boundary it would probably win.

> Perhaps we still let the app fill the bitmap?

I think I'd want to see some performance data on it first.