Re: [PATCH v2 14/18] KVM: arm64: spe: Provide guest virtual interrupts for SPE

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On 2019-12-24 13:36, Andrew Murray wrote:
On Tue, Dec 24, 2019 at 01:22:46PM +0000, Marc Zyngier wrote:
On 2019-12-24 13:08, Andrew Murray wrote:

[...]

> This does feel like the pragmatic approach - a larger black hole in
> exchange
> for performance. I imagine the blackhole would be naturally reduced on
> machines with high workloads.

Why? I don't see the relation between how busy the vcpu is and the size of the blackhole. It is strictly a function of the frequency of exits.

Indeed, my assumption being that the busier a system is the more
interrupts, thus leading to more exits and so an increased frequency of
SPE interrupt evaluation and thus smaller black hole.

On a GICv4-enabled system, this isn't true anymore. My bet is that
people won't use SPE to optimize IO-oriented workloads, but more CPU
intensive workloads (that don't necessarily exit at all).

But never mind. Let's start with this approach, as it is simple and easy
to verify. If the black hole aspect becomes problematic, we know how
to reduce it (at the expense of entry/exit performance).

        M.
--
Jazz is not dead. It just smells funny...
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