Re: [PATCH v2 3/6] arm64/kvm: disable access to AMU registers from kvm guests

[Valentin Schneider]

On 18/12/2019 18:26, Ionela Voinescu wrote:
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index 6e5d839f42b5..dd20fb185d56 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -266,10 +266,11 @@
>  #define CPTR_EL2_TFP_SHIFT 10
>  
>  /* Hyp Coprocessor Trap Register */
> -#define CPTR_EL2_TCPAC	(1 << 31)
> -#define CPTR_EL2_TTA	(1 << 20)
> -#define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
>  #define CPTR_EL2_TZ	(1 << 8)
> +#define CPTR_EL2_TFP	(1 << CPTR_EL2_TFP_SHIFT)
> +#define CPTR_EL2_TTA	(1 << 20)
> +#define CPTR_EL2_TAM	(1 << 30)
> +#define CPTR_EL2_TCPAC	(1 << 31)

Nit: why the #define movement? Couldn't that just be added beneath
CPTR_EL2_TCPAC?

>  #define CPTR_EL2_RES1	0x000032ff /* known RES1 bits in CPTR_EL2 */
>  #define CPTR_EL2_DEFAULT	CPTR_EL2_RES1
>  
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 72fbbd86eb5e..0bca87a2621f 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -90,6 +90,17 @@ static void activate_traps_vhe(struct kvm_vcpu *vcpu)
>  	val = read_sysreg(cpacr_el1);
>  	val |= CPACR_EL1_TTA;
>  	val &= ~CPACR_EL1_ZEN;
> +
> +	/*
> +	 * With VHE enabled, we have HCR_EL2.{E2H,TGE} = {1,1}. Note that in
> +	 * this case CPACR_EL1 has the same bit layout as CPTR_EL2, and
> +	 * CPACR_EL1 accessing instructions are redefined to access CPTR_EL2.
> +	 * Therefore use CPTR_EL2.TAM bit reference to activate AMU register
> +	 * traps.
> +	 */
> +
> +	val |= CPTR_EL2_TAM;
> +

Hmm so this is a bit confusing for me, I've rewritten that part of the
email too many times (didn't help that I'm far from being a virt guru).
Rectifications are most welcome.


First, AFAICT we *don't* have HCR_EL2.TGE set anymore at this point, it's
cleared just a bit earlier in __activate_traps().


Then, your comment suggests that when we're running this code, CPACR_EL1
accesses are rerouted to CPTR_EL2. Annoyingly this isn't mentioned in
the doc of CPACR_EL1, but D5.6.3 does say

"""
When ARMv8.1-VHE is implemented, and HCR_EL2.E2H is set to 1, when executing
at EL2, some EL1 System register access instructions are redefined to access
the equivalent EL2 register.
"""

And CPACR_EL1 is part of these, so far so good. Now, the thing is
the doc for CPACR_EL1 *doesn't* mention any TAM bit - but CPTR_EL2 does.
I believe what *do* want here is to set CPTR_EL2.TAM (which IIUC we end
up doing via the rerouting).

So, providing I didn't get completely lost on the way, I have to ask:
why do we use CPACR_EL1 here? Couldn't we use CPTR_EL2 directly?


>  	if (update_fp_enabled(vcpu)) {
>  		if (vcpu_has_sve(vcpu))
>  			val |= CPACR_EL1_ZEN;
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 9f2165937f7d..940ab9b4c98b 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1003,6 +1003,20 @@ static bool access_pmuserenr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
>  	{ SYS_DESC(SYS_PMEVTYPERn_EL0(n)),					\
>  	  access_pmu_evtyper, reset_unknown, (PMEVTYPER0_EL0 + n), }
>  
> +static bool access_amu(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
> +			     const struct sys_reg_desc *r)
> +{
> +	kvm_inject_undefined(vcpu);
> +
> +	return false;
> +}
> +
> +/* Macro to expand the AMU counter and type registers*/
> +#define AMU_AMEVCNTR0_EL0(n) { SYS_DESC(SYS_AMEVCNTR0_EL0(n)), access_amu }
> +#define AMU_AMEVTYPE0_EL0(n) { SYS_DESC(SYS_AMEVTYPE0_EL0(n)), access_amu }
> +#define AMU_AMEVCNTR1_EL0(n) { SYS_DESC(SYS_AMEVCNTR1_EL0(n)), access_amu }
> +#define AMU_AMEVTYPE1_EL0(n) { SYS_DESC(SYS_AMEVTYPE1_EL0(n)), access_amu }
> +

You could save a *whopping* two lines with something like:

#define AMU_AMEVCNTR_EL0(group, n) { SYS_DESC(SYS_AMEVCNTR##group##_EL0(n)), access_amu }
#define AMU_AMEVTYPE_EL0(group, n) { SYS_DESC(SYS_AMEVTYPE##group##_EL0(n)), access_amu }

Though it doesn't help shortening the big register list below.

>  static bool trap_ptrauth(struct kvm_vcpu *vcpu,
>  			 struct sys_reg_params *p,
>  			 const struct sys_reg_desc *rd)
> @@ -1078,8 +1092,12 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
>  			 (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
>  	u64 val = raz ? 0 : read_sanitised_ftr_reg(id);
>  
> -	if (id == SYS_ID_AA64PFR0_EL1 && !vcpu_has_sve(vcpu)) {
> -		val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> +	if (id == SYS_ID_AA64PFR0_EL1) {
> +		if (!vcpu_has_sve(vcpu))
> +			val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> +		val &= ~(0xfUL << ID_AA64PFR0_AMU_SHIFT);
> +	} else if (id == SYS_ID_PFR0_EL1) {
> +		val &= ~(0xfUL << ID_PFR0_AMU_SHIFT);
>  	} else if (id == SYS_ID_AA64ISAR1_EL1 && !vcpu_has_ptrauth(vcpu)) {
>  		val &= ~((0xfUL << ID_AA64ISAR1_APA_SHIFT) |
>  			 (0xfUL << ID_AA64ISAR1_API_SHIFT) |

Could almost turn the thing into a switch case at this point.