Re: [PATCH 6/8] KVM: arm64: Add sys_reg_descs for remaining MTE CMO instructions

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

On Wed, Nov 04, 2026 at 11:21:22AM -0800, Oliver Upton wrote:
> Filtering unsupported cache maintenance instructions from a guest relies
> on either having a suitable FGU bit for the feature or a descriptor with
> a visibility() hook.
> 
> MTE CMO instructions can be trapped by HCR_EL2.TPCP and lack a suitable
> FGU. Add descriptors for the CMO instructions that UNDEF if MTE isn't
> implemented for the VM.
> 

I think I get the idea of using MTE_CMO_INSN(): having the visibility test 
for MTE defined in the previous patch, which makes sense with the above 
commit message: if !mte -> hidden, then on access it injects undefined.

Please help me understand the other way: if we have mte, then it calls the 
'access' callback, which is bad_cmo_trap(), which also inject undefined. 
What is the goal here?

Thanks!
Leo



> Signed-off-by: Oliver Upton <oupton@kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index b45635471344..7f7a197a9ad6 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -4295,6 +4295,12 @@ static bool handle_tlbi_el1(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
>  		.access = (access_fn),					\
>  	}
>  
> +static bool bad_cmo_trap(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
> +			 const struct sys_reg_desc *r)
> +{
> +	return bad_trap(vcpu, p, r, "MTE CMO trap");
> +}
> +
>  #define MTE_CMO_INSN(insn, access_fn)					\
>  	{								\
>  		SYS_DESC(SYS_##insn),					\
> @@ -4304,7 +4310,9 @@ static bool handle_tlbi_el1(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
>  
>  static struct sys_reg_desc sys_insn_descs[] = {
>  	{ SYS_DESC(SYS_DC_ISW), access_dcsw },
> +	MTE_CMO_INSN(DC_IGVAC, bad_cmo_trap),
>  	MTE_CMO_INSN(DC_IGSW, access_dcsw),
> +	MTE_CMO_INSN(DC_IGDVAC, bad_cmo_trap),
>  	MTE_CMO_INSN(DC_IGDSW, access_dcsw),
>  
>  	SYS_INSN(AT_S1E1R, handle_at_s1e01),
> @@ -4321,6 +4329,7 @@ static struct sys_reg_desc sys_insn_descs[] = {
>  	{ SYS_DESC(SYS_DC_CISW), access_dcsw },
>  	MTE_CMO_INSN(DC_CIGSW, access_dcsw),
>  	MTE_CMO_INSN(DC_CIGDSW, access_dcsw),
> +	MTE_CMO_INSN(DC_CIGDVAPS, bad_cmo_trap),
>  
>  	SYS_INSN(TLBI_VMALLE1OS, handle_tlbi_el1),
>  	SYS_INSN(TLBI_VAE1OS, handle_tlbi_el1),
> @@ -4394,6 +4403,15 @@ static struct sys_reg_desc sys_insn_descs[] = {
>  	SYS_INSN(TLBI_VALE1NXS, handle_tlbi_el1),
>  	SYS_INSN(TLBI_VAALE1NXS, handle_tlbi_el1),
>  
> +	MTE_CMO_INSN(DC_CGVAC, bad_cmo_trap),
> +	MTE_CMO_INSN(DC_CGDVAC, bad_cmo_trap),
> +	MTE_CMO_INSN(DC_CGVAP, bad_cmo_trap),
> +	MTE_CMO_INSN(DC_CGDVAP, bad_cmo_trap),
> +	MTE_CMO_INSN(DC_CGVADP, bad_cmo_trap),
> +	MTE_CMO_INSN(DC_CGDVADP, bad_cmo_trap),
> +	MTE_CMO_INSN(DC_CIGVAC, bad_cmo_trap),
> +	MTE_CMO_INSN(DC_CIGDVAC, bad_cmo_trap),
> +
>  	SYS_INSN(AT_S1E2R, handle_at_s1e2),
>  	SYS_INSN(AT_S1E2W, handle_at_s1e2),
>  	SYS_INSN(AT_S12E1R, handle_at_s12),
> -- 
> 2.47.3
>