Re: vtime accounting

[Marc Zyngier <marc.zyngier@arm.com>]

On Tue, Mar 14 2017 at  8:26:01 am GMT, Christoffer Dall <cdall@linaro.org> wrote:
> On ARM, I think the main benefits of implementing something like
> handle_external_intr would come from two things: (1) You'd avoid the
> context synchronization and associated cost of taking an exception on
> the CPU, and (2) you'd also (potentially) avoid the additional
> save/restore of all the GP regiters from the kernel exception entry path
> to create a usable gp_regs.
>
> I have to look more careful at whether or not (2) is possible, because
> it would mean we'd have to store the guest register state on a pt_regs
> structure in the first place and pass that directly to
> arch_handle_irq.

For that to be useful on pre-VHE HW and avoid the extra exception
handling, we'd also have to perform that arch_handle_irq call as part of
the exception return to EL1. That's not going to be very pretty, as we
need to build two return contexts (EL2->EL1 IRQ on the host, followed by
EL1->EL1).

This is all perfectly doable, but as always, we need numbers.

I had similar stuff for VHE a long while ago (back when I wrote WSINC),
but faking the pt_regs was absolutely horrible. But maybe it is time to
exhume this again...

> Additionally, if we had something like handle_external_intr, the
> guest_exit thing would be kinda moot, since we'd do our ticks like
> x86...

Indeed. Which brings the obvious question: are we the only ones not
taking the interrupt early?

Thanks,

	M.
-- 
Jazz is not dead, it just smell funny.