linux-kernel - Re: [PATCH] arm64: Add kernel parameter to disable trap EL0 accesses to IMPDEF regs

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Message-ID: <865xc7wcjw.wl-maz@kernel.org>
Date: Wed, 22 Oct 2025 09:44:35 +0100
From: Marc Zyngier <maz@...nel.org>
To: "Liao, Chang" <liaochang1@...wei.com>
Cc: <corbet@....net>,
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Subject: Re: [PATCH] arm64: Add kernel parameter to disable trap EL0 accesses to IMPDEF regs

On Wed, 22 Oct 2025 03:37:04 +0100,
"Liao, Chang" <liaochang1@...wei.com> wrote:
> 
> Hi Marc,
> 
> 在 2025/10/21 20:25, Marc Zyngier 写道:
> > On Tue, 21 Oct 2025 12:54:28 +0100,
> > Liao Chang <liaochang1@...wei.com> wrote:
> >>
> >> Add kernel parameter to allow system-wide EL0 access to IMPDEF system
> >> regregisters and instructions without trapping to EL1/EL2. Since trap
> >> overhead will compromises benefits, and it's even worse in
> >> virtualization on CPU where certain IMPDEF registers and instructions
> >> are designed for EL0 performance use.
> > 
> > Since you mention virtualisation, I want to be clear: there is no way
> > I will consider anything like this for KVM. KVM will always trap and
> > UNDEF such register accesses, no matter where they come from (EL0 or
> > EL1).
> > 
> > Allowing such registers to be accessed from within a guest would make
> > it impossible to context-switch or save/restore the guest correctly.
> > 
> > You can of course do what you want in your downstream kernel or your
> > own hypervisor, but I wanted to set the expectations on the upstream
> > side.
> 
> Does it make sense to allow EL0 access IMPDEF without trapping for some
> special vendor CPUID, instead of forbidding it as the default setting on
> the upstream code?

Let me answer your question with my own questions:

How can supervisory software (kernel or hypervisor) save and restore
state that it doesn't know about? How can it ensure isolation of state
if there are unspecified registers that can change unspecified things
behind its back?

You'd need to add CPU-specific code to the kernel to make that work,
and SW written to make use of these functionalities wouldn't work
anywhere else. So if you end-up with a custom userspace, why should
upstream care? There is zero benefit to the ecosystem.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.