Re: [RFC 5/8] KVM: arm64: Explicitly handle MDSELR_EL1 traps as UNDEFINED

[Marc Zyngier <maz@kernel.org>]

On Fri, 12 Apr 2024 03:41:23 +0100,
Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
> 
> 
> On 4/5/24 15:45, Marc Zyngier wrote:
> > On Fri, 05 Apr 2024 09:00:05 +0100,
> > Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> >>
> >> Currently read_sanitised_id_aa64dfr0_el1() caps the ID_AA64DFR0.DebugVer to
> >> ID_AA64DFR0_DebugVer_V8P8, resulting in FEAT_Debugv8p9 not being exposed to
> >> the guest. MDSELR_EL1 register access in the guest, is currently trapped by
> >> the existing configuration of the fine-grained traps.
> > 
> > Please add support for the HDFGxTR2_EL2 registers in the trap routing
> > arrays, add support for the corresponding FGUs in the corresponding
> 
> Afraid that I might not have enough background here to sufficiently understand
> your suggestion above, but nonetheless here is an attempt in this regard.

Thanks for at least giving it a try, this is *MUCH* appreciated.

>
> - Add HDFGRTR2_EL2/HDFGWTR2_EL2 to enum vcpu_sysreg
> 	enum vcpu_sysreg {
> 		..........
> 		VNCR(HDFGRTR2_EL2),
> 		VNCR(HDFGWTR2_EL2),
> 		..........
> 	}

Yes.

> 
> - Add their VNCR mappings addresses
> 
> 	#define VNCR_HDFGRTR2_EL2      0x1A0
> 	#define VNCR_HDFGWTR2_EL2      0x1B0

Yes.

> 
> - Add HDFGRTR2_EL2/HDFGWTR2_EL2 to sys_reg_descs[]
> 
> static const struct sys_reg_desc sys_reg_descs[] = {
> 	..........
> 	EL2_REG_VNCR(HDFGRTR2_EL2, reset_val, 0),
> 	EL2_REG_VNCR(HDFGWTR2_EL2, reset_val, 0),
> 	..........
> }

Yes

> 
> - Add HDFGRTR2_GROUP to enum fgt_group_id
> - Add HDFGRTR2_GROUP to reg_to_fgt_group_id()
> - Update triage_sysreg_trap() for HDFGRTR2_GROUP
> - Update __activate_traps_hfgxtr() both for HDFGRTR2_EL2 and HDFGWTR2_EL2
> - Updated __deactivate_traps_hfgxtr() both for HDFGRTR2_EL2 and HDFGWTR2_EL2

Yes. Don't miss check_fgt_bit() though.  You also need to update
kvm_init_nv_sysregs() to ensure that these new registers have the
correct RES0/RES1 behaviour depending on the supported feature set for
the guest.

>
> > structure, and condition the UNDEF on the lack of *guest* support for
> > the feature.
> 
> Does something like the following looks OK for preventing guest access into
> MDSELR_EL1 instead ?
> 
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1711,6 +1711,19 @@ static u64 read_sanitised_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
>         return val;
>  }
>  
> +static bool trap_mdselr_el1(struct kvm_vcpu *vcpu,
> +                          struct sys_reg_params *p,
> +                          const struct sys_reg_desc *r)
> +{
> +       u64 dfr0 = read_sanitised_id_aa64dfr0_el1(vcpu, r);
> +       int dver = cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_DebugVer_SHIFT);
> +
> +       if (dver != ID_AA64DFR0_EL1_DebugVer_V8P9)
> +               return undef_access(vcpu, p, r);

This is very cumbersome, and we now have a much better infrastructure
for the stuff that is handled with FGTs, see below.

> +
> +       return true;
> +}
> +
>  static int set_id_aa64dfr0_el1(struct kvm_vcpu *vcpu,
>                                const struct sys_reg_desc *rd,
>                                u64 val)
> @@ -2203,7 +2216,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>         { SYS_DESC(SYS_MDSCR_EL1), trap_debug_regs, reset_val, MDSCR_EL1, 0 },
>         DBG_BCR_BVR_WCR_WVR_EL1(2),
>         DBG_BCR_BVR_WCR_WVR_EL1(3),
> -       { SYS_DESC(SYS_MDSELR_EL1), undef_access },
> +       { SYS_DESC(SYS_MDSELR_EL1), trap_mdselr_el1 },
>         DBG_BCR_BVR_WCR_WVR_EL1(4),
>         DBG_BCR_BVR_WCR_WVR_EL1(5),
>         DBG_BCR_BVR_WCR_WVR_EL1(6),
> 
> I am sure this is rather incomplete, but will really appreciate if you could
> provide some details and pointers.

What is missing is the Fine-Grained-Undef part. You need to update
kvm_init_sysreg() so that kvm->arch.fgu[HDFGRTR2_GROUP] has all the
correct bits set for anything that needs to UNDEF depending on the
guest configuration.

For example, in your case, I'd expect to see something like:

if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, DebugVer, V8P9))
	kvm->arch.fgu[HDFGRTR2_GROUP] |= ~(HDFGRTR2_EL2_nMDSELR_EL1 | [...]);

Then allowing the feature becomes conditioned on the bit being clear,
and the trap handler only needs to deal with the actual emulation, and
not the feature checking.

I appreciate that this is a lot to swallow, but I'd be very happy to
review patches implementing this and provide guidance. It is all
pretty simple, just that there is a lot of parts all over the place.
In the end, this is only about following the architecture.

Thanks again,

	M.

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