linux-kernel - Re: [RFC, PATCHv1 15/28] x86: detect 5-level paging support

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Date:   Tue, 13 Dec 2016 14:50:21 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Borislav Petkov <bp@...en8.de>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC, PATCHv1 15/28] x86: detect 5-level paging support

On 12/09/16 07:32, Kirill A. Shutemov wrote:
> 
> Something like this?
> 
> diff --git a/arch/x86/boot/cpuflags.c b/arch/x86/boot/cpuflags.c
> index 6687ab953257..366aad972025 100644
> --- a/arch/x86/boot/cpuflags.c
> +++ b/arch/x86/boot/cpuflags.c
> @@ -70,16 +70,22 @@ int has_eflag(unsigned long mask)
>  # define EBX_REG "=b"
>  #endif
> 
> -static inline void cpuid(u32 id, u32 *a, u32 *b, u32 *c, u32 *d)
> +static inline void cpuid_count(u32 id, u32 count,
> +               u32 *a, u32 *b, u32 *c, u32 *d)
>  {
> +       *a = id;
> +       *c = count;

These two lines are wrong, remove them.

>         asm volatile(".ifnc %%ebx,%3 ; movl  %%ebx,%3 ; .endif  \n\t"
>                      "cpuid                                     \n\t"
>                      ".ifnc %%ebx,%3 ; xchgl %%ebx,%3 ; .endif  \n\t"
>                     : "=a" (*a), "=c" (*c), "=d" (*d), EBX_REG (*b)
> -                   : "a" (id)
> +                   : "a" (id), "c" (count)
>         );
>  }
> 
> +#define cpuid(id, a, b, c, d) cpuid_count(id, 0, a, b, c, d)
> +

Other than that, it's correct.

That being said, the claim that ECX ought to be zeroed on a
non-subleaf-equipped CPUID leaf is spurious, in my opinion.  That being
said, it also doesn't do any harm and might avoid problems in the
opposite direction, e.g. someone thinking that leaf 7 doesn't have
subleaves.

It might also be better to have something like:

#define SAVE_EBX(x) ".ifnc %%ebx," x "; movl %%ebx," x "; .endif"
#define SWAP_EBX(x) ".ifnc %%ebx," x "; xchgl %%ebx," x "; .endif"

... but if it is only used once it might just be more confusion.

	-hpa