Re: [RFC][PATCH] svm.c: tsc unsigned delta calculation

[Anthony Liguori <anthony@codemonkey.ws>]

Ryan Harper wrote:
> I've been playing around with smp guests on a couple amd systems and
> I've also seen some of the smp/locking issues which lead me to dig into
> some of the tsc code.  The svm_cpu_load, the tsc_offset calc will
> generate a massively large tsc_offset if we're switching cpus and
> tsc_this is ahead of the host_tsc value (delta would normally be
> negative, but since it's unsigned, we get a huge positive number).
>
> svm_vcpu_load()
> ...
>     rdtscll(tsc_this);
>     delta = vcpu->arch.host_tsc - tsc_this;
>     svm->vmcb->control.tsc_offset += delta;
>   

This math will work out fine since the very large number will generate 
an overflow and the result will be identical to if we were using s64s.  
We're using u64s because that's how the tsc_offset is defined by hardware.

> This is handled a little differently on Intel (in vmx.c) where there is
> a check:
>
>     if (tsc_this < vcpu->arch.host_tsc)
>         /* do delta and new offset calc */
>   

So what your patch really does is change the behavior of the tsc_offset 
to increase the guest's TSC by a potentially large amount depending on 
how out of sync the TSC is on CPU migration.

The question is why this would make things work out better for you..

Do you have Gerd's kvm-clock most recent patch applied?

Regards,

Anthony Liguori

> This check makes sense to me in that we only need to ensure that we
> don't go backwards which means that unless the new cpu is behind the
> current vcpu's host_tsc, we can skip a new delta calc.  If the check
> doesn't make sense then we'll need to do the math with s64s.
>
> Attached patch fixed the case where an idle guest was live-locked.
>
>   


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