RE: [Xen-ia64-devel]PATCH] Remove duplicate check is_running_on_xen

["Xu, Anthony" <anthony.xu@xxxxxxxxx>]

Alex Williamson write on 2007年1月17日 1:02:
> On Mon, 2007-01-15 at 13:33 +0800, Xu, Anthony wrote:
>> 
>> Remove duplicate check is_running_on_xen
> 
> Hi Anthony,
> 
>    I'm not sure I understand why the xen functions, for example
> xen_get_psr(), need to support both bare metal and paravirtualized
> since the caller always checks for is_running_on_xen.  The current
> code seems overly paranoid.  Would is make more sense to rename these
> to __xen_get_psr() and eliminate the running_on_xen check in the
> assembly? Then we could still use the native bare metal calls when
> running a Xen kernel on bare metal.  Am I missing some reason why
> xen_get_psr() would ever get called on bare metal?  Thanks,

There are two duplicate checks as below, so there are two way to eliminate one 
check.

1. eliminate check in xen_get_tpr, then xen_get_tpr will never be called on 
bare metal,
     but it's a little perfomance impact, due to there is another switch inside 
__ia64_getreg.

2. eliminate check in xen_ia64_getreg, then xen_get_tpr will be called both on 
para platform
   or bare metal, it can produce better performance than option 1. so I prefer 
this.

Anthony


#define xen_ia64_getreg(regnum)                                         \

        case _IA64_REG_CR_IVR:                                          \
                ia64_intri_res = (is_running_on_xen()) ?                        
\
                        xen_get_ivr() :                                 \
                        __ia64_getreg(regnum);                          \
                break;                                                  \

GLOBAL_ENTRY(xen_get_tpr)
        movl r8=running_on_xen;;
        ld4 r8=[r8];;
        cmp.eq p7,p0=r8,r0;;
(p7)    mov r8=cr.tpr;;
(p7)    br.ret.sptk.many rp
        ;;
        XEN_CLEAR_PSR_IC
        ;;
        XEN_HYPER_GET_TPR
        ;;
        XEN_RESTORE_PSR_IC
        ;;
        br.ret.sptk.many rp
        ;;
END(xen_get_tpr)





> 
>       Alex
> 
>> diff -r 29780963b34f linux-2.6-xen-sparse/arch/ia64/xen/hypercall.S
>> --- a/linux-2.6-xen-sparse/arch/ia64/xen/hypercall.S    Mon Jan 15
>> 04:27:37 2007 +0800 +++
>> b/linux-2.6-xen-sparse/arch/ia64/xen/hypercall.S    Mon Jan 15
>>  05:02:05 2007 +0800 @@ -11,12 +11,9 @@ GLOBAL_ENTRY(xen_get_psr)
>>         GLOBAL_ENTRY(xen_get_psr) movl r8=running_on_xen;;
>>         ld4 r8=[r8];;
>> -       cmp.eq p7,p0=r8,r0;;
>> -(p7)   mov r8=psr;;
>> -(p7)   br.ret.sptk.many rp
>> -       ;;
>> -       XEN_HYPER_GET_PSR
>> -       ;;
>> +       cmp.eq p7,p6=r8,r0;;
>> +(p7)   mov r8=psr
>> +(p6)   XEN_HYPER_GET_PSR
>>         br.ret.sptk.many rp
>>         ;;
>>  END(xen_get_psr)
> ...
>>  linux-2.6-xen-sparse/include/asm-ia64/xen/privop.h
>> --- a/linux-2.6-xen-sparse/include/asm-ia64/xen/privop.h        Mon
>> Jan 15 04:27:37 2007 +0800
>> +++ b/linux-2.6-xen-sparse/include/asm-ia64/xen/privop.h        Mon
>> Jan 15 05:08:12 2007 +0800
>> @@ -203,24 +203,16 @@ extern void xen_ptcga(unsigned long addr
>>                                                                     
>> \         switch(regnum) {                                          
>>         \ case
>> _IA64_REG_PSR:                                             \
>> -               ia64_intri_res =
>> (is_running_on_xen()) ?                        \
>> -                       xen_get_psr() :                             
>> \ 
>> -                       __ia64_getreg(regnum);                      
>> \ +               ia64_intri_res =
>> xen_get_psr();                                 \
>>                 break;                                              
>> \ 

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