Re: [PATCH 8/8] x86/AMD: Clean up context_update() in AMD VPMU code

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 04/11/2013 03:48 PM, Suravee Suthikulpanit wrote:
> Boris,
>
> I have a couple questions.  After patched, here is the final code:
>
> static int amd_vpmu_do_rdmsr(unsigned int msr, uint64_t *msr_content)
> {
>     struct vcpu *v = current;
>     struct vpmu_struct *vpmu = vcpu_vpmu(v);
>
>     if ( vpmu_is_set(vpmu, VPMU_STOPPED) )
>     {
>         context_load(v);
>         vpmu_reset(vpmu, VPMU_STOPPED);
>     }
>
>     if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_LOADED) )
>         context_read(msr, msr_content);
>     else
>         rdmsrl(msr, *msr_content);
>
>     return 1;
> }
>
> 1. Why do you need to call context_load() here since you checking if 
> the we are in VPMU_CONTEXT_LOADED and get the appropriate copy before 
> returning the msr_content?

It is possible to be both LOADED and STOPPED. Example: guest writes 
control register (but does not enable the counters, so VPMU is not 
RUNNING). In this case amd_vpmu_load() will not get called from 
vpmu_load() so subsequent rdmsr of this control register in the guest 
will need to read from context and not from HW (since HW register is zero).

In this case context_load() really only needs to load control registers; 
the counters themselves are the same in both context and HW.

> 2. If you have already called context_load() above, shouldn't you have 
> to call "vpmu_set(vpmu, VPMU_CONTEXT_LOADED)" as well?  But then, you 
> wouldn't be needing the latter logic, isn't it?


Yes, you are right. I think we can get rid if context_read() completely 
and simply load the context if it is not LOADED or is STOPPED. Just like 
it is done in amd_vpmu_do_wrmsr().

-boris