Re: [Xen-devel] [PATCH v4 04/15] x86: implement data structure and CPU init flow for MBA

[Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>]

On 17-10-03 23:52:09, Jan Beulich wrote:
> >>> Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx> 09/29/17 3:55 AM >>>
> >On 17-09-28 05:00:09, Jan Beulich wrote:
> >> >>> On 23.09.17 at 11:48, <yi.y.sun@xxxxxxxxxxxxxxx> wrote:
> >> > One thrtl register of MBA stores a throttle value for one or more
> >> > domains. The throttle value means the delay between L2 cache and next
> >> > cache level.
> >> 
> >> What is a delay between two cache levels?
> >> 
> >There is a "programmable rate controller" between them to indirectly control
> >the bandwidth.
> 
> I'm afraid I don't feel like this answers my question. Anyway - the sentence
> needs some re-phrasing so it becomes clear what you're talking about.
> 
I may draw the figure shown in SDM to make things clear.

> >> > @@ -1410,6 +1496,7 @@ static void psr_cpu_init(void)
> >> >      unsigned int socket, cpu = smp_processor_id();
> >> >      struct feat_node *feat;
> >> >      struct cpuid_leaf regs;
> >> > +    uint32_t ebx;
> >> 
> >> Is this local variable really a big help? To me it looks like it only
> >> makes the patch larger without actually improving anything,
> >> and without being related to the subject of the patch.
> >> 
> >IMHO, it can avoid the 'cpuid_count_leaf()' being repeatedly called. Without 
> >it,
> >we have to call 'cpuid_count_leaf()' for 2 more times.
> 
> Hmm, didn't you simply replace regs.b uses with ebx? Or did I overlook a place
> where regs is being overwritten before the last of these regs.b uses (in 
> which case
> I think your change is fine)?
> 
The regs is overwritten when a feature presents. The old codes are below

    cpuid_count_leaf(PSR_CPUID_LEVEL_CAT, 0, &regs);
    if ( regs.b & PSR_RESOURCE_TYPE_L3 )
    {
        cpuid_count_leaf(PSR_CPUID_LEVEL_CAT, 1, &regs); //It is overwritten 
here.
......
    }

    cpuid_count_leaf(PSR_CPUID_LEVEL_CAT, 0, &regs);  //So, we have to call 
cpuid to get regs again.
    if ( regs.b & PSR_RESOURCE_TYPE_L2 )
    {
        cpuid_count_leaf(PSR_CPUID_LEVEL_CAT, 2, &regs);
......

Because above reason, I defined this ebx local variable to avoid calling cpuid
again for next feature.

> Jan

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