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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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
> 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. 

Do you have a full patchset somewhere for testing?
> 
> --Andy

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