linux-kernel - Re: [PATCH 3/8] x86: Add intrinsics/macros for new rd/wr fs/gs base instructions

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Date:	Fri, 10 Apr 2015 12:07:52 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	X86 ML <x86@...nel.org>, Andrew Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 3/8] x86: Add intrinsics/macros for new rd/wr fs/gs base instructions

On Fri, Apr 10, 2015 at 8:50 AM, Andi Kleen <andi@...stfloor.org> wrote:
> From: Andi Kleen <ak@...ux.intel.com>
>
> Add C intrinsics and assembler macros for the new rd/wr fs/gs base
> instructions and for swapgs.
>
> Very straight forward. Used in followon patch.
>
> For assembler only a few standard registers used by entry_64.S
> are defined.
>
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/include/asm/fsgs.h | 54 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 54 insertions(+)
>  create mode 100644 arch/x86/include/asm/fsgs.h
>
> diff --git a/arch/x86/include/asm/fsgs.h b/arch/x86/include/asm/fsgs.h
> new file mode 100644
> index 0000000..1df5085
> --- /dev/null
> +++ b/arch/x86/include/asm/fsgs.h
> @@ -0,0 +1,54 @@
> +#ifndef _ASM_FSGS_H
> +#define _ASM_FSGS_H 1
> +
> +#ifndef __ASSEMBLY__
> +
> +static inline void swapgs(void)
> +{
> +       asm volatile("swapgs" ::: "memory");
> +}
> +
> +/* Must be protected by X86_FEATURE_FSGSBASE check. */
> +
> +static inline unsigned long rdgsbase(void)
> +{
> +       unsigned long gs;
> +       asm volatile(".byte 0xf3,0x48,0x0f,0xae,0xc8 # rdgsbaseq %%rax"
> +                       : "=a" (gs)
> +                       :: "memory");
> +       return gs;
> +}
> +
> +static inline unsigned long rdfsbase(void)
> +{
> +       unsigned long fs;
> +       asm volatile(".byte 0xf3,0x48,0x0f,0xae,0xc0 # rdfsbaseq %%rax"
> +                       : "=a" (fs)
> +                       :: "memory");
> +       return fs;
> +}
> +
> +static inline void wrgsbase(unsigned long gs)
> +{
> +       asm volatile(".byte 0xf3,0x48,0x0f,0xae,0xd8 # wrgsbaseq %%rax"
> +                       :: "a" (gs)
> +                       : "memory");
> +}
> +
> +static inline void wrfsbase(unsigned long fs)
> +{
> +       asm volatile(".byte 0xf3,0x48,0x0f,0xae,0xd0 # wrfsbaseq %%rax"
> +                       :: "a" (fs)
> +                       : "memory");
> +}

I think these should be always_inline.  If they somehow fail to get
inlined and then get kprobed or traced, bad things will happen.

--Andy
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