linux-kernel - Re: [PATCH -tip v3 1/2] x86: xen: insn: Decode Xen and KVM emulate-prefix signature

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Date:   Fri, 6 Sep 2019 09:34:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH -tip v3 1/2] x86: xen: insn: Decode Xen and KVM
 emulate-prefix signature

On Fri, Sep 06, 2019 at 10:45:48AM +0900, Masami Hiramatsu wrote:

> diff --git a/arch/x86/include/asm/xen/interface.h b/arch/x86/include/asm/xen/interface.h
> index 62ca03ef5c65..fe33a9798708 100644
> --- a/arch/x86/include/asm/xen/interface.h
> +++ b/arch/x86/include/asm/xen/interface.h
> @@ -379,12 +379,15 @@ struct xen_pmu_arch {
>   * Prefix forces emulation of some non-trapping instructions.
>   * Currently only CPUID.
>   */
> +#include <asm/xen/prefix.h>
> +
>  #ifdef __ASSEMBLY__
> -#define XEN_EMULATE_PREFIX .byte 0x0f,0x0b,0x78,0x65,0x6e ;
> +#define XEN_EMULATE_PREFIX .byte __XEN_EMULATE_PREFIX ;
>  #define XEN_CPUID          XEN_EMULATE_PREFIX cpuid
>  #else
> -#define XEN_EMULATE_PREFIX ".byte 0x0f,0x0b,0x78,0x65,0x6e ; "
> +#define XEN_EMULATE_PREFIX ".byte " __XEN_EMULATE_PREFIX_STR " ; "
>  #define XEN_CPUID          XEN_EMULATE_PREFIX "cpuid"
> +
>  #endif

Possibly you can do something like:

#define XEN_EMULATE_PREFIX	__ASM_FORM(.byte __XEN_EMULATE_PREFIX ;)
#define XEN_CPUID		XEN_EMULATE_PREFIX __ASM_FORM(cpuid)

>  #endif /* _ASM_X86_XEN_INTERFACE_H */
> diff --git a/arch/x86/include/asm/xen/prefix.h b/arch/x86/include/asm/xen/prefix.h
> new file mode 100644
> index 000000000000..f901be0d7a95
> --- /dev/null
> +++ b/arch/x86/include/asm/xen/prefix.h
> @@ -0,0 +1,10 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _TOOLS_ASM_X86_XEN_PREFIX_H
> +#define _TOOLS_ASM_X86_XEN_PREFIX_H
> +
> +#include <linux/stringify.h>
> +
> +#define __XEN_EMULATE_PREFIX  0x0f,0x0b,0x78,0x65,0x6e
> +#define __XEN_EMULATE_PREFIX_STR  __stringify(__XEN_EMULATE_PREFIX)
> +
> +#endif

How about we make this asm/virt_prefix.h or something and include:

/*
 * Virt escape sequences to trigger instruction emulation;
 * ideally these would decode to 'whole' instruction and not destroy
 * the instruction stream; sadly this is not true for the 'kvm' one :/
 */

#define __XEN_EMULATE_PREFIX  0x0f,0x0b,0x78,0x65,0x6e  /* ud2 ; .ascii "xen" */
#define __KVM_EMULATE_PREFIX  0x0f,0x0b,0x6b,0x76,0x6d	/* ud2 ; .ascii "kvm" */

> diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
> index 0b5862ba6a75..b7eb50187db9 100644
> --- a/arch/x86/lib/insn.c
> +++ b/arch/x86/lib/insn.c

> @@ -58,6 +61,37 @@ void insn_init(struct insn *insn, const void *kaddr, int buf_len, int x86_64)
>  		insn->addr_bytes = 4;
>  }
>  
> +static const insn_byte_t xen_prefix[] = { __XEN_EMULATE_PREFIX };
> +/* See handle_ud()@arch/x86/kvm/x86.c */
> +static const insn_byte_t kvm_prefix[] = "\xf\xbkvm";

Then you can make this consistent; maybe even something like:

static const insn_byte_t *virt_prefix[] = {
	{ __XEN_EMULATE_PREFIX },
	{ __KVM_EMULATE_PREFIX },
	{ NULL },
};

And then change emulate_prefix_size to emulate_prefix_index ?