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Message-ID: <1b961c71d30e31ecb22da2c5401b1a81cb802d86.camel@intel.com>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Dave Hansen <dave.hansen@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>,
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        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>
Date:   Mon, 10 Jun 2019 15:40:49 -0700
In-Reply-To: <92e56b28-0cd4-e3f4-867b-639d9b98b86c@intel.com>
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On Mon, 2019-06-10 at 15:02 -0700, Dave Hansen wrote:
> On 6/10/19 1:58 PM, Yu-cheng Yu wrote:
> > > > On each memory request, the kernel then must consider a percentage of
> > > > allocated space in its calculation, and on systems with less memory
> > > > this quickly becomes a problem.
> > > 
> > > I'm not sure what you're referring to here?  Are you referring to our
> > > overcommit limits?
> > 
> > Yes.
> 
> My assumption has always been that these large, potentially sparse
> hardware tables *must* be mmap()'d with MAP_NORESERVE specified.  That
> should keep them from being problematic with respect to overcommit.

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.

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

Perhaps we still let the app fill the bitmap?

Yu-cheng