linux-kernel - Re: [RFC PATCH] KVM: arm64: vgic-v3: Cache ICC_CTLR

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Message-ID: <CAFEAcA8VL4wE2w9PegPD_AVSKCZzzzx-jFru7dDOq4Tz5xs76A@mail.gmail.com>
Date: Tue, 14 Oct 2025 10:31:28 +0100
From: Peter Maydell <peter.maydell@...aro.org>
To: Salil Mehta <salil.mehta@...src.net>
Cc: Marc Zyngier <maz@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, salil.mehta@...wei.com, 
	jonathan.cameron@...wei.com, will@...nel.org, catalin.marinas@....com, 
	mark.rutland@....com, james.morse@....com, sudeep.holla@....com, 
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Subject: Re: [RFC PATCH] KVM: arm64: vgic-v3: Cache ICC_CTLR_EL1 and allow
 lockless read when ready

On Tue, 14 Oct 2025 at 04:02, Salil Mehta <salil.mehta@...src.net> wrote:
> On Mon, Oct 13, 2025 at 4:48 PM Peter Maydell <peter.maydell@...aro.org> wrote:
> > That said, QEMU's current code for this could be refactored
> > to avoid the reset-time read of ICC_CTLR_EL1 from the kernel.
> > We do this so we can set the userspace struct field for this
> > register to the right value. But we could ask the kernel for
> > that value once on VM startup since it's not going to change mid-run.
> >
> > That would bring ICC_CTLR_EL1 into line with the other cpuif
> > registers, where QEMU assumes it knows what the kernel's
> > reset value of them is (mostly "0") and doesn't bother to ask.
> > This is different from how we handle ONE_REG sysregs, where
> > I'm pretty sure we do ask the kernel the value of all of them
> > on a vcpu reset. (And then write the values back again, which
> > is a bit silly but nobody's ever said it was a performance
> > problem for them :-))
>
>
> This is effectively what the mentioned patch in the commit-log is doing.
> Pasting here again:
>
> https://lore.kernel.org/qemu-devel/20251001010127.3092631-22-salil.mehta@opnsrc.net/

No, that is not what I have in mind. What I mean is that
we can just read the ICC_CTLR_EL1 value once on startup
(in our realize method, I think) and then use that value in
reset. That should not require any of that machinery to pause
the VM and keep retrying.

-- PMM