Re: [PATCH 3/3] x86_64,vsyscall: Make vsyscall emulation configurable

[Konrad Rzeszutek Wilk]

On Wed, Oct 29, 2014 at 02:30:29PM -0700, Andy Lutomirski wrote:
> On Oct 29, 2014 1:00 PM, "Konrad Rzeszutek Wilk" <konrad.wilk@xxxxxxxxxx> wrote:
> >
> > On Tue, Oct 28, 2014 at 11:09:53AM -0700, Andy Lutomirski wrote:
> > > On Tue, Oct 28, 2014 at 10:57 AM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> > > > On Tue, Oct 28, 2014 at 10:22:28AM -0700, Andy Lutomirski wrote:
> > > >> This adds CONFIG_X86_VSYSCALL_EMULATION, guarded by CONFIG_EXPERT.
> > > >> Turning it off completely disables vsyscall emulation, saving ~3.5k
> > > >> for vsyscall_64.c, 4k for vsyscall_emu_64.S (the fake vsyscall
> > > >> page), some tiny amount of core mm code that supports a gate area,
> > > >> and possibly 4k for a wasted pagetable.  The latter is because the
> > > >> vsyscall addresses are misaligned and fit poorly in the fixmap.
> > > >>
> > > >> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > > >
> > > > One minor nit below, but with or without that change,
> > > > Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > > >
> > > >> --- a/arch/x86/xen/mmu.c
> > > >> +++ b/arch/x86/xen/mmu.c
> > > >> @@ -1456,11 +1456,13 @@ static int xen_pgd_alloc(struct mm_struct *mm)
> > > >>               user_pgd = (pgd_t *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
> > > >>               page->private = (unsigned long)user_pgd;
> > > >>
> > > >> +#ifdef CONFIG_X86_VSYSCALL_EMULATION
> > > >>               if (user_pgd != NULL) {
> > > >>                       user_pgd[pgd_index(VSYSCALL_ADDR)] =
> > > >>                               __pgd(__pa(level3_user_vsyscall) | _PAGE_TABLE);
> > > >>                       ret = 0;
> > > >>               }
> > > >> +#endif
> > > >
> > > > Could you instead make the if use IS_ENABLED?
> > > >
> > > >                 if (IS_ENABLED(CONFIG_X86_VSYSCALL_EMULATION) && user_pgd != NULL)
> > > >
> > > > That has the advantage of ensuring that the code continues to compile.
> > > > (Given that you haven't removed level3_user_vsyscall, that should work.)
> > >
> > > I need the ret = 0, I think, so I'll resend.
> > >
> > > I think I'd rather use #ifdef here, since I think it would be great if
> > > the Xen people could clean this up further.  With this change, under
> > > some configurations, there should be no user-accessible kernel
> > > addresses at all.  (Also, is there some PV mechanism

What about the vsyscall time stamp (aka kvm-clock). That is not
really VSYSCALL emulation based but normal code?

> > > that I'm not thinking of that will break with this change?  I know
> > > I've tripped over Xen pagetable and fixmap oddities before.)
> >
> > Not that I know of. The vsyscall is the only one that I know of that
> > does this.
> 
> There's kvm-clock, too, but that may never co-exist with Xen.

It will eventually.
> 
> I tagged v2 here:
> 
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/tag/?id=optional-vsyscall-emulation-v2
> 
> and I'll send it out in a bit.
> 
> --Andy
> 
> >
> > Do you have a full patchset somewhere for testing?
> > >
> > > --Andy
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