Messages in this thread | | | From | Andy Lutomirski <> | Date | Sun, 11 Nov 2018 21:34:42 -0800 | Subject | Re: [PATCH RFC 0/3] Static calls |
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On Sun, Nov 11, 2018 at 9:02 PM Ingo Molnar <mingo@kernel.org> wrote: > > > * Josh Poimboeuf <jpoimboe@redhat.com> wrote: > > > On Fri, Nov 09, 2018 at 08:28:11AM +0100, Ingo Molnar wrote: > > > > - I'm not sure about the objtool approach. Objtool is (currently) > > > > x86-64 only, which means we have to use the "unoptimized" version > > > > everywhere else. I may experiment with a GCC plugin instead. > > > > > > I'd prefer the objtool approach. It's a pretty reliable first-principles > > > approach while GCC plugin would have to be replicated for Clang and any > > > other compilers, etc. > > > > The benefit of a plugin is that we'd only need two of them: GCC and > > Clang. And presumably, they'd share a lot of code. > > > > The prospect of porting objtool to all architectures is going to be much > > more of a daunting task (though we are at least already considering it > > for some arches). > > Which architectures would benefit from ORC support the most? > > I really think that hard reliance on GCC plugins is foolish - but maybe > Clang's plugin infrastructure is a guarantee that it remains a sane and > usable interface. > > > > I'd be very happy with a demonstrated paravirt optimization already - > > > i.e. seeing the before/after effect on the vmlinux with an x86 distro > > > config. > > > > > > All major Linux distributions enable CONFIG_PARAVIRT=y and > > > CONFIG_PARAVIRT_XXL=y on x86 at the moment, so optimizing it away as much > > > as possible in the 99.999% cases where it's not used is a primary > > > concern. > > > > For paravirt, I was thinking of it as more of a cleanup than an > > optimization. The paravirt patching code already replaces indirect > > branches with direct ones -- see paravirt_patch_default(). > > > > Though it *would* reduce the instruction footprint a bit, as the 7-byte > > indirect calls (later patched to 5-byte direct + 2-byte nop) would > > instead be 5-byte direct calls to begin with. > > Yes.
It would be a huge cleanup IMO -- the existing PVOP call stuff is really quite ugly IMO. Also, the existing stuff tries to emulate the semantics of passing parameters of unknown types using asm constraints, and I just don't believe that GCC does what we want it to do. In general, passing the *value* of a pointer to asm doesn't seem to convince gcc that the pointed-to value is used by the asm, and this makes me nervous. See commit 715bd9d12f84d8f5cc8ad21d888f9bc304a8eb0b as an example of this. In a similar vein, the existing PVOP calls have a "memory" clobber, and that's not free.
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