Re: [PATCH v2 19/28] arm64: Add command line switch to disable FEAT_S2FWB

[Leonardo Bras <leo.bras@arm.com>]

On Fri, Oct 23, 2026 at 01:56:01PM -0700, Oliver Upton wrote:
> Assigning noncoherent devices to KVM guests does not work when guest
> memory is forced writeback. There's absolutely no reason to support
> this configuration, especially when disabling FWB is a valid fallback.
> 
> Add a command line switch to disable FWB in anticipation of KVM
> mandating it for noncoherent DMA. Use a kvm_arm alias that can be
> remapped in the distant future to select passthough S2 attributes
> when FEAT_S2FWB becomes mandatory in the architecture.
> 
> Signed-off-by: Oliver Upton <oupton@kernel.org>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 4 ++++
>  arch/arm64/kernel/pi/idreg-override.c           | 2 ++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 33cd30996e47..1c0e3b9e726d 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3265,6 +3265,10 @@ Kernel parameters
>  			"nested" and "protected" are experimental and should be
>  			used with extreme caution.
>  
> +	kvm-arm.nofwb
> +			[KVM,ARM,EARLY] Disable the use of stage-2 forced
> +			writeback in KVM.
> +
>  	kvm-arm.vgic_v3_group0_trap=
>  			[KVM,ARM,EARLY] Trap guest accesses to GICv3 group-0
>  			system registers
> diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
> index 274bc72ba98d..188e1fb2eb2c 100644
> --- a/arch/arm64/kernel/pi/idreg-override.c
> +++ b/arch/arm64/kernel/pi/idreg-override.c
> @@ -103,6 +103,7 @@ static const struct ftr_set_desc mmfr2 __prel64_initconst = {
>  	.override	= &id_aa64mmfr2_override,
>  	.fields		= {
>  		FIELD("varange", ID_AA64MMFR2_EL1_VARange_SHIFT, mmfr2_varange_filter),
> +		FIELD("fwb", ID_AA64MMFR2_EL1_FWB_SHIFT, NULL),
>  		{}
>  	},
>  };
> @@ -245,6 +246,7 @@ static const struct {
>  } aliases[] __initconst = {
>  	{ "kvm_arm.mode=nvhe",		"arm64_sw.hvhe=0 id_aa64mmfr1.vh=0" },
>  	{ "kvm_arm.mode=protected",	"arm64_sw.hvhe=1" },
> +	{ "kvm_arm.nofwb",		"id_aa64mmfr2.fwb=0" },
>  	{ "arm64.nosve",		"id_aa64pfr0.sve=0" },
>  	{ "arm64.nosme",		"id_aa64pfr1.sme=0" },
>  	{ "arm64.nobti",		"id_aa64pfr1.bt=0" },
> -- 
> 2.47.3
> 

Oh, did not know about this idreg_override feature, really cool.
Patch looks straightforward.

FWIW:
Reviewed-by: Leonardo Bras <leo.bras@arm.com>

Thanks!
Leo