From: Thomas Garnier <thgarnie@google.com>
Subject: Re: x86: PIE support and option to extend KASLR randomization
Date: Fri, 22 Sep 2017 11:59:00 -0700
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Cc: Ingo Molnar <mingo@kernel.org>, 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 Machek <pavel@ucw.cz>, Tejun Heo <tj@kernel.org>,
	Christoph Lamet
To: "H. Peter Anvin" <hpa@zytor.com>
In-Reply-To: <b65e912f-eeb3-5f1e-bb4a-2dffc0b6481e@zytor.com>

On Fri, Sep 22, 2017 at 11:38 AM, H. Peter Anvin <hpa@zytor.com> wrote:
> 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.

We have a GOT with very few entries, mainly linker script globals that
I think we can work to reduce or remove.

We have a PLT but it is empty. On latest iteration (not sent yet),
modules have PLT32 relocations but no PLT entry. I got rid of
mcmodel=large for modules and instead I move the beginning of the
module section just after the kernel so relative relocations work.

>
>         -hpa


-- 
Thomas