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

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Date:	Tue, 28 Oct 2014 10:57:38 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org, mingo@...nel.org
Subject: Re: [PATCH 3/3] x86_64,vsyscall: Make vsyscall emulation configurable

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@...capital.net>

One minor nit below, but with or without that change,
Reviewed-by: Josh Triplett <josh@...htriplett.org>

> --- 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.)

- Josh Triplett
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