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Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function
To:     "H.J. Lu" <hjl.tools@gmail.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Peter Zijlstra <peterz@infradead.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <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>,
        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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Message-ID: <46043aaf-7fd8-2aa1-2306-24de5bff64bf@intel.com>
Date:   Mon, 10 Jun 2019 16:37:09 -0700
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On 6/10/19 4:20 PM, H.J. Lu wrote:
>>> Perhaps we still let the app fill the bitmap?
>> I think I'd want to see some performance data on it first.
> Updating legacy bitmap in user space from kernel requires
> 
> long q;
> 
> get_user(q, ...);
> q |= mask;
> put_user(q, ...);
> 
> instead of
> 
> *p |= mask;
> 
> get_user + put_user was quite slow when we tried before.

Numbers, please.

There are *lots* of ways to speed something like that up if you have
actual issues with it.  For instance, you can skip the get_user() for
whole bytes.  You can write bits with 0's for unallocated address space.
 You can do user_access_begin/end() to avoid bunches of STAC/CLACs...

The list goes on and on. :)