linux-kernel - Re: [PATCH] Documentation: Update the behaviour of "kvm-arm.mode"

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Message-ID: <ZxowSYHQeOq7W_JT@google.com>
Date: Thu, 24 Oct 2024 11:32:25 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Marc Zyngier <maz@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, corbet@....net,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH] Documentation: Update the behaviour of "kvm-arm.mode"

Hi Marc,

On Thu, Oct 24, 2024 at 10:39:45AM +0100, Marc Zyngier wrote:
> Hi Mostafa,
> 
> On Wed, 23 Oct 2024 18:12:43 +0100,
> Mostafa Saleh <smostafa@...gle.com> wrote:
> > 
> > Commit 5053c3f0519c ("KVM: arm64: Use hVHE in pKVM by default on CPUs with
> > VHE support") modified the behaviour of "kvm-arm.mode=protected" without
> > the updating the kernel parameters doc.
> > 
> > Update it to match the current implementation.
> > 
> > Cc: Will Deacon <will@...nel.org>
> > Cc: Marc Zyngier <maz@...nel.org>
> > 
> > Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index bb48ae24ae69..59a0dd7e2de6 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -2723,8 +2723,12 @@
> >  			nvhe: Standard nVHE-based mode, without support for
> >  			      protected guests.
> >  
> > -			protected: nVHE-based mode with support for guests whose
> > +			protected: hVHE-based mode with support for guests whose
> >  				   state is kept private from the host.
> > +				   In case hVHE is not supported in hardware, it will
> 
> nit: it is VHE that is supported or not, hVHE is only a SW concept.
> 
> > +				   boot with protected nVHE.
> > +				   nVHE protected mode can still be forced on VHE systems
> > +				   using "kvm_arm.mode=protected arm64_sw.hvhe=0 id_aa64mmfr1.vh=0"
> 
> This opens another question: none of the arm_sw.*, nor any of the
> id_aa64* parameters are described (basically, anything that's in
> arch/arm64/kernel/pi/id_override.c). What should we do about these?

Yes, I mainly added this, to make it easier if someone wants to boot to
protected nVHE so they don't have to go through the code, but I can
remove it if it's confusing.

> 
> 
> 
> >
> >  			nested: VHE-based mode with support for nested
> >  				virtualization. Requires at least ARMv8.3
> 
> Huh, another nit to fix. We only support nested with ARMv8.4 (with
> FEAT_NV2), as the ARMv8.3 version (the original FEAT_NV) is too ugly
> for words.
> 
> Mind addressing this?

Sure, I will update it in v2.

Thanks,
Mostafa
> 
> Thanks!
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.