Re: [Qemu-devel] [PATCH v2] Build *-user targets as PIE

[Kirill A. Shutemov]

On Thu, Sep 3, 2009 at 8:17 PM, Reimar
Döffinger<address@hidden> wrote:
> On Thu, Sep 03, 2009 at 06:07:21PM +0300, Kirill A. Shutemov wrote:
>> On Thu, Sep 3, 2009 at 5:38 PM, Paul Brook<address@hidden> wrote:
>> >> PIE code usually is a bit slower. Approximately, 1% for i386 according to
>> >> some tests. RISC architectures should be affected less, since they have
>> >> more registers. On other hand we are getting rid from text relocations on
>> >> i386 which make executable loading slower. So...
>> >
>> > I think you've got that backwards.
>> > A traditional (fixed address) executable requires no load-time relocation 
>> > for
>> > internal references because all addresses are known at static link time. 
>> > PIE
>> > require the dynamic linker adjust all absolute addresses.
>>
>> Usermode qemu on i386 is not a traditional executable, sicne it uses
>> -Wl,-shared for linking. In result we've got an executable which looks
>> like PIE, but dynamic linker have to resolve text relocations. I think
>> the best way it to create a true PIE without a text relocations.
>
> It is close to getting of topic, but since you state it, why try so hard
> to avoid text relocations?
> Sure, there are advantages (the biggest one is less issues with
> mis-/insufficiently configured selinux I think), possibly better sharing
> of pages when many instances are run and better delayed loading, but on
> x86/i386 that doesn't sound like a clear advantage compared to the in
> some cases quite relevant speed loss.

Do you have any numbers about speed loss?

> Also, since this patch adds --disable-pie, isn't the hack currently used
> still necessary for that case?
> Or is --disable-pie supposed to disable self-hosting? Then maybe the
> option should be named --disable-self-hosting (and if that is indeed the
> only side-effect it might be better to disable it by default)?
>