Re: [Xen-devel] [PATCH 3/3] x86: Use alternative mechanism to define CLAC/STAC

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Andrew Cooper [mailto:amc96@xxxxxxxxxxxxxxxx] On Behalf Of
> Andrew Cooper
> Sent: Monday, May 26, 2014 11:50 PM
> To: Wu, Feng; xen-devel@xxxxxxxxxxxxx
> Cc: keir.xen@xxxxxxxxx; JBeulich@xxxxxxxx
> Subject: Re: [Xen-devel] [PATCH 3/3] x86: Use alternative mechanism to define
> CLAC/STAC
> 
> On 26/05/2014 08:27, Feng Wu wrote:
> > This patch use alternative mechanism to define CLAC/STAC.
> >
> > Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> > ---
> >  xen/include/asm-x86/asm_defns.h | 36
> +++++++++++++++++++++++-------------
> >  1 file changed, 23 insertions(+), 13 deletions(-)
> >
> > diff --git a/xen/include/asm-x86/asm_defns.h
> b/xen/include/asm-x86/asm_defns.h
> > index 87a462f..92024cc 100644
> > --- a/xen/include/asm-x86/asm_defns.h
> > +++ b/xen/include/asm-x86/asm_defns.h
> > @@ -10,6 +10,8 @@
> >  #include <asm/percpu.h>
> >  #include <xen/stringify.h>
> >  #include <asm/cpufeature.h>
> > +#include <asm/nops.h>
> > +#include <asm/alternative.h>
> 
> alternatives should include nops, to avoid needing a double include
> everywhere alternative() is used.
> 
> >
> >  #ifndef __ASSEMBLY__
> >  void ret_from_intr(void);
> > @@ -166,26 +168,34 @@ void ret_from_intr(void);
> >  #define __ASM_STAC      .byte 0x0f,0x01,0xcb
> >
> >  #ifdef __ASSEMBLY__
> > -#define ASM_AC(op)                                       \
> > -        btl $X86_FEATURE_SMAP & 31,                      \
> > -
> CPUINFO_FEATURE_OFFSET(X86_FEATURE_SMAP)+boot_cpu_data(%rip); \
> > -        jnc 881f;                                        \
> > -        __ASM_##op;                                      \
> > -881:
> > -
> > -#define ASM_STAC ASM_AC(STAC)
> > -#define ASM_CLAC ASM_AC(CLAC)
> > +#define ASM_CLAC
> \
> > +        661: ASM_NOP3;
> \
> > +        .pushsection .altinstr_replacement, "ax";
> \
> 
> The altinstr_replacement section doesn't need to be executable.  The
> data inside it is copied out into the .text section.
> 
> > +        662: __ASM_CLAC;
> \
> > +        .popsection;
> \
> > +        .pushsection .altinstructions, "a";
> \
> > +        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;
> \
> > +        .popsection
> 
> these 661s and 662s alias magic numbers in the alternatives core code,
> which could lead to subtle issues.  Surely there is a better way than
> having to choose magic numbers which don't collide with anything else in
> the source.
> 
> ~Andrew

Do you think what problems would be caused by these magic numbers? Since
these macros are ported from Linux being unchanged and they have already been
running for a long time, I think it should be safe enough for us to reuse here.

Thanks,
Feng

> 
> > +
> > +#define ASM_STAC
> \
> > +        661: ASM_NOP3;
> \
> > +        .pushsection .altinstr_replacement, "ax";
> \
> > +        662: __ASM_STAC;
> \
> > +        .popsection;
> \
> > +        .pushsection .altinstructions, "a";
> \
> > +        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;
> \
> > +        .popsection
> >  #else
> >  static inline void clac(void)
> >  {
> > -    if ( boot_cpu_has(X86_FEATURE_SMAP) )
> > -        asm volatile (__stringify(__ASM_CLAC) : : : "memory");
> > +    /* Note: a barrier is implicit in alternative() */
> > +    alternative(ASM_NOP3, __stringify(__ASM_CLAC),
> X86_FEATURE_SMAP);
> >  }
> >
> >  static inline void stac(void)
> >  {
> > -    if ( boot_cpu_has(X86_FEATURE_SMAP) )
> > -        asm volatile (__stringify(__ASM_STAC) : : : "memory");
> > +    /* Note: a barrier is implicit in alternative() */
> > +    alternative(ASM_NOP3, __stringify(__ASM_STAC),
> X86_FEATURE_SMAP);
> >  }
> >  #endif
> >


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel