From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: x86: PIE support and option to extend KASLR randomization
Date: Fri, 22 Sep 2017 11:38:05 -0700
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Cc: Herbert Xu <herbert@gondor.apana.org.au>,
        "David S . Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        Matthias Kaehlcke <mka@chromium.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross
 <jgross@suse.com>, Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Joerg Roedel <joro@8bytes.org>, Tom Lendacky <thomas.lendacky@amd.com>,
        Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
        Brian Gerst <brgerst@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Len Brown <len.brown@intel.com>, Pavel Mach
To: Ingo Molnar <mingo@kernel.org>, Thomas Garnier <thgarnie@google.com>
In-Reply-To: <20170922163225.bfrd5myl6d7deiim@gmail.com>
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On 09/22/17 09:32, Ingo Molnar wrote:
> 
> BTW., I think things improved with ORC because with ORC we have RBP as an extra 
> register and with PIE we lose RBX - so register pressure in code generation is 
> lower.
> 

We lose EBX on 32 bits, but we don't lose RBX on 64 bits - since x86-64
has RIP-relative addressing there is no need for a dedicated PIC register.

I'm somewhat confused how we can have as much as almost 1% overhead.  I
suspect that we end up making a GOT and maybe even a PLT for no good reason.

	-hpa