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From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Mon, 10 Jun 2019 16:20:52 -0700
Message-ID: <CAMe9rOqLxNxE-gGX9ozX=emW9iQ+gOeUiS3ec5W4jmF6wk6cng@mail.gmail.com>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function
To:     Dave Hansen <dave.hansen@intel.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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On Mon, Jun 10, 2019 at 3:59 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> > 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.

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.

-- 
H.J.