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

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Message-ID: <86v7xh3km6.wl-maz@kernel.org>
Date: Thu, 24 Oct 2024 10:39:45 +0100
From: Marc Zyngier <maz@...nel.org>
To: Mostafa Saleh <smostafa@...gle.com>
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 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?



>
>  			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?

Thanks!

	M.

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