Re: [Xen-devel] [PATCH] nestedhvm: ASID emulation

[Christoph Egger <Christoph.Egger@xxxxxxx>]

On 04/13/11 17:05, Keir Fraser wrote:
> On 13/04/2011 15:26, "Christoph Egger"<Christoph.Egger@xxxxxxx>  wrote:
>
> > On 04/13/11 15:27, Keir Fraser wrote:
> > > On 13/04/2011 11:37, "Christoph Egger"<Christoph.Egger@xxxxxxx>   wrote:
> > We talk about a win of about 1000 cycles per VMRUN and another 1000
> > cycles per VMEXIT emulation.
> >
> > That's a speedup of about 10% for each VMRUN and about 20% for each
> > VMEXIT emulation.
> Is this measurable on a macro benchmark?
I measured this with xentrace while l2 guest is booting.

The speedup is noticable on the end-user side just by the feeling on how 
fast the l2 guest reacts on user input.

> I mean this looks like a micro-optimisation on a feature that noone is going
> to use for serious performance work anyway.
>
> > 4. nestedhvm is enabled and we are going to run l2 guest
> >
> > We run the l1 guest in the last call. The asid generation may have
> > changed by then. In this case the current nv_n2asid number is stale
> > and the value of data->next_asid is<= of nv->nv_n2asid.
> How do you know for sure that next_asid will be<= nv_n2asid in this case?
> What if other VCPUs have run meanwhile, and next_asid has been incremented
> multiple times until it is greather than nv_n2asid?
In that case  curr->arch.hvm_vcpu.asid_generation is not equal to 
data->core_asid_generation  and a new hw ASID number is assigned
regardless of the values of data->next_asid and nv_n2asid.

Christoph

>   -- Keir
>
> > The the value of nv->nv_n2asid is valid if l1 guest doesn't change
> > the virtual asid (= asid number in the virtual vmcb) and
> > data->next_asid is larger than nv->nv_n2asid. In this case
> > just reuse the same hw ASID that has been used from the last
> > VMRUN emulation.
> >
> > 5. nestedhvm is enabled and we are going to run l2 guest again
> >
> > The same hw ASID should be reused unless the generation changed because
> > the nestedp2m got flushed or the vcpu moved to a different physical cpu,
> > for example.
> > But the hw ASID number may never match the hw ASID used to run the l1 guest.
> >
> >
> > In all cases we have to verify that the
> >
> > > I wouldn't bother fixing #2 unless there's a convincing answer for #1.
> > >
> > >    -- Keir

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