Re: [PATCH v6 15/64] KVM: arm64: nv: Handle HCR_EL2.E2H specially

Marc Zyngier
2022-02-01T18:17:23Z

On 2022-02-01 16:51, Russell King (Oracle) wrote:
> On Fri, Jan 28, 2022 at 12:18:23PM +0000, Marc Zyngier wrote:
>> HCR_EL2.E2H is nasty, as a flip of this bit completely changes the way
>> we deal with a lot of the state. So when the guest flips this bit
>> (sysregs are live), do the put/load dance so that we have a consistent
>> state.
>>
>> Yes, this is slow. Don't do it.
>
> I'd hope this is very unlikely!

A guest OS would probably do it once per CPU bring-up. So I'm
not too bothered about the speed. But that's only one of the
many cases where we need to do this put/load game.

At this stage, we don't care too much. But the last two patches
give you a glimpse of what sort of fine-grained optimisation
we will eventually want to do for this not to suck too much.

But again, this is NV, and it gives a whole new sense to "being
slow".

>
>>
>> Suggested-by: Alexandru Elisei <alexandru.elisei@arm.com>
>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>
> Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Thanks,

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