* Thomas Garnier <thgar...@google.com> wrote:
> > > -model=small/medium assume you are on the low 32-bit. It generates
> > > instructions where the virtual addresses have the high 32-bit to be zero.
> >
> > How are these assumptions hardcoded by GCC? Most of the instructions should
> > be
> > relocatable straight away, as most call/jump/branch instructions are
> > RIP-relative.
>
> I think PIE is capable to use relative instructions well. mcmodel=large
> assumes
> symbols can be anywhere.
So if the numbers in your changelog and Kconfig text cannot be trusted, there's
this description of the size impact which I suspect is less susceptible to
measurement error:
+ The kernel and modules will generate slightly more assembly (1 to 2%
+ increase on the .text sections). The vmlinux binary will be
+ significantly smaller due to less relocations.
... but describing a 1-2% kernel text size increase as "slightly more assembly"
shows a gratituous disregard to kernel code generation quality! In reality
that's
a huge size increase that in most cases will almost directly transfer to a 1-2%
slowdown for kernel intense workloads.
Where does that size increase come from, if PIE is capable of using relative
instructins well? Does it come from the loss of a generic register and the
resulting increase in register pressure, stack spills, etc.?
So I'm still unhappy about this all, and about the attitude surrounding it.
Thanks,
Ingo
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel
