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

On 04/11/2013 03:48 PM, Suravee Suthikulpanit wrote:

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) )
        vpmu_reset(vpmu, VPMU_STOPPED);

    if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_LOADED) )
        context_read(msr, msr_content);
        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().


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