Re: [PATCH] lguest-fix-divide-error-implement-sched_clock

["H. Peter Anvin" <hpa@zytor.com>]

Andi Kleen wrote:
> 
> I don't think it's a good idea for the TSC. There are various
> setups where it is unreliable and also often simulators don't 
> implement it correctly. And it's always a valuable workaround
> to be able to turn it off.
> 
> Except possibly for the FPU only features used by the gcc output
> should be tested this way. For everything else it is better to 
> test at runtime.
> 
> That is x86-64 makes some more assumptions. But even it
> doesn't assume TSC. 
> 
> I added the mechanism to statically evaluate mostly to share cpufeatures.h
> between 32bit and 64bit at some point -- but didn't quite finish that work 
> before the last merge.
> 

Note that tsc_setup in i386 at least has:


static int __init tsc_setup(char *str)
{
        printk(KERN_WARNING "notsc: Kernel compiled with CONFIG_X86_TSC,"
                                "cannot disable TSC.\n");
        return 1;
}


If TSC isn't actually a compile-time feature then we should allow it to
be disabled on the command line!


This is what I have for i386:

#ifndef CONFIG_MATH_EMULATION
# define NEED_FPU       (1<<(X86_FEATURE_FPU & 31))
#else
# define NEED_FPU       0
#endif

#ifdef CONFIG_X86_TSC
# define NEED_TSC       (1<<(X86_FEATURE_TSC & 31))
#else
# define NEED_TSC       0
#endif

#ifdef CONFIG_X86_PAE
# define NEED_PAE       (1<<(X86_FEATURE_PAE & 31))
#else
# define NEED_PAE       0
#endif

#ifdef CONFIG_X86_CMOV
# define NEED_CMOV      (1<<(X86_FEATURE_CMOV & 31))
#else
# define NEED_CMOV      0
#endif

#ifdef CONFIG_X86_CMPXCHG64
# define NEED_CX8       (1<<(X86_FEATURE_CX8 & 31))
#else
# define NEED_CX8       0
#endif

#define REQUIRED_MASK0  (NEED_FPU|NEED_TSC|NEED_PAE|NEED_CMOV|NEED_CX8)

#ifdef CONFIG_X86_USE_3DNOW
# define NEED_3DNOW     (1<<(X86_FEATURE_3DNOW & 31))
#else
# define NEED_3DNOW     0
#endif

#define REQUIRED_MASK1  (NEED_3DNOW)

And for x86-64:


/* x86-64 baseline features */
#define NEED_FPU        (1<<(X86_FEATURE_FPU & 31))
#define NEED_PSE        (1<<(X86_FEATURE_PSE & 31))
#define NEED_TSC        (1<<(X86_FEATURE_TSC & 31))
#define NEED_MSR        (1<<(X86_FEATURE_MSR & 31))
#define NEED_PAE        (1<<(X86_FEATURE_PAE & 31))
#define NEED_CX8        (1<<(X86_FEATURE_CX8 & 31))
#define NEED_PGE        (1<<(X86_FEATURE_PGE & 31))
#define NEED_FXSR       (1<<(X86_FEATURE_FXSR & 31))
#define NEED_CMOV       (1<<(X86_FEATURE_CMOV & 31))
#define NEED_XMM        (1<<(X86_FEATURE_XMM & 31))
#define NEED_XMM2       (1<<(X86_FEATURE_XMM2 & 31))

#define REQUIRED_MASK0  (NEED_FPU|NEED_PSE|NEED_TSC|NEED_MSR|NEED_PAE|\
                         NEED_CX8|NEED_PGE|NEED_FXSR|NEED_CMOV|\
                         NEED_XMM|NEED_XMM2)
#define SSE_MASK        (NEED_XMM|NEED_XMM2)

/* x86-64 baseline features */
#define NEED_LM         (1<<(X86_FEATURE_LM & 31))

#ifdef CONFIG_X86_USE_3DNOW
# define NEED_3DNOW     (1<<(X86_FEATURE_3DNOW & 31))
#else
# define NEED_3DNOW     0
#endif

#define REQUIRED_MASK1  (NEED_LM|NEED_3DNOW)