Re: [PATCH 04/11] KVM: arm64: CCA: Check for LPA2 support and SHA-256

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] KVM: arm64: rmi_has_feature() passes non-constant mask to u64_get_bits()
--

commit a9400749495394b73c9cccd919996b48771556fe
Author: Steven Price <steven.price@arm.com>

KVM: arm64: CCA: Check for LPA2 support and SHA-256

This commit adds feature checks during KVM RMI initialization to verify that
the RMM supports LPA2 when KVM has LPA2 enabled and that the RMM supports
SHA-256 measurements.

> diff --git a/arch/arm64/kvm/rmi.c b/arch/arm64/kvm/rmi.c
> index 9d118768550d6..5b07beff8bc91 100644
> --- a/arch/arm64/kvm/rmi.c
> +++ b/arch/arm64/kvm/rmi.c
> @@ -6,8 +6,30 @@
>  #include <linux/arm-rmi-cmds.h>
>  #include <linux/kvm_host.h>
>  
> +#include <asm/kvm_pgtable.h>
>  #include <asm/virt.h>
>  
> +static bool rmi_has_feature(int reg, unsigned long feature)
> +{
> +	return !!u64_get_bits(rmi_feat_reg(reg), feature);
> +}

[Severity: Low]
Can this fail to build if the compiler decides not to inline
rmi_has_feature()?

In include/linux/bitfield-fix-width.h, u64_get_bits() calls
field_multiplier(field):

static __always_inline
u64 field_multiplier(u64 field)
{
	if ((field | (field - 1)) & ((field | (field - 1)) + 1))
		__bad_mask();
	return field & -field;
}

Because rmi_has_feature() is not marked __always_inline, if the compiler does
not inline it (for example under CONFIG_CC_OPTIMIZE_FOR_SIZE or compiler
instrumentation), the feature parameter is not a compile-time constant inside
field_multiplier(), preventing constant folding from eliminating the call to
__bad_mask().

Should rmi_has_feature() be marked __always_inline, or use field_get() or a
direct bitwise test instead?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261102141304.126883-1-steven.price@arm.com?part=4