Subject: Re: [PATCH 13/59] KVM: arm64: nv: Handle virtual EL2 registers in vcpu_read/write_sys_reg()

From: Marc Zyngier <[email-masked]>
Date: Wed, 3 Jul 2019 16:59:02 +0100

On 25/06/2019 16:18, Alexandru Elisei wrote:
> Hi Marc,
> 
> A question regarding this patch. This patch modifies vcpu_{read,write}_sys_reg
> to handle virtual EL2 registers. However, the file kvm/emulate-nested.c added by
> patch 10/59 "KVM: arm64: nv: Support virtual EL2 exceptions" already uses
> vcpu_{read,write}_sys_reg to access EL2 registers. In my opinion, it doesn't
> really matter which comes first because nested support is only enabled in the
> last patch of the series, but I thought I should bring this up in case it is not
> what you intended.

It doesn't really matter at that stage. The only thing I'm trying to
achieve in the middle of the series is not to break the build, and not
to cause non-NV to fail.

> 
> On 6/21/19 10:37 AM, Marc Zyngier wrote:
>> From: Andre Przywara <[email-masked]>
>>
>> KVM internally uses accessor functions when reading or writing the
>> guest's system registers. This takes care of accessing either the stored
>> copy or using the "live" EL1 system registers when the host uses VHE.
>>
>> With the introduction of virtual EL2 we add a bunch of EL2 system
>> registers, which now must also be taken care of:
>> - If the guest is running in vEL2, and we access an EL1 sysreg, we must
>>   revert to the stored version of that, and not use the CPU's copy.
>> - If the guest is running in vEL1, and we access an EL2 sysreg, we must
>>   also use the stored version, since the CPU carries the EL1 copy.
>> - Some EL2 system registers are supposed to affect the current execution
>>   of the system, so we need to put them into their respective EL1
>>   counterparts. For this we need to define a mapping between the two.
>>   This is done using the newly introduced struct el2_sysreg_map.
>> - Some EL2 system registers have a different format than their EL1
>>   counterpart, so we need to translate them before writing them to the
>>   CPU. This is done using an (optional) translate function in the map.
>> - There are the three special registers SP_EL2, SPSR_EL2 and ELR_EL2,
>>   which need some separate handling.
> I see no change in this patch related to SPSR_EL2. Special handling of SPSR_EL2
> is added in the next patch, patch 14/59 "KVM: arm64: nv: Handle SPSR_EL2 specially".

Indeed, this needs rewriting (we ended-up splitting the SPSR stuff out
as it was messy and not completely correct). I may take the rest of the
special stuff out as well.

>>
>> All of these cases are now wrapped into the existing accessor functions,
>> so KVM users wouldn't need to care whether they access EL2 or EL1
>> registers and also which state the guest is in.
>>
>> This handles what was formerly known as the "shadow state" dynamically,
>> without requiring a separate copy for each vCPU EL.
>>
>> Signed-off-by: Andre Przywara <[email-masked]>
>> Signed-off-by: Marc Zyngier <[email-masked]>
>> ---
>>  arch/arm64/include/asm/kvm_emulate.h |   6 +
>>  arch/arm64/include/asm/kvm_host.h    |   5 +
>>  arch/arm64/kvm/sys_regs.c            | 163 +++++++++++++++++++++++++++
>>  3 files changed, 174 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
>> index c43aac5fed69..f37006b6eec4 100644
>> --- a/arch/arm64/include/asm/kvm_emulate.h
>> +++ b/arch/arm64/include/asm/kvm_emulate.h
>> @@ -70,6 +70,12 @@ void kvm_emulate_nested_eret(struct kvm_vcpu *vcpu);
>>  int kvm_inject_nested_sync(struct kvm_vcpu *vcpu, u64 esr_el2);
>>  int kvm_inject_nested_irq(struct kvm_vcpu *vcpu);
>>  
>> +u64 translate_tcr(u64 tcr);
>> +u64 translate_cptr(u64 tcr);
>> +u64 translate_sctlr(u64 tcr);
>> +u64 translate_ttbr0(u64 tcr);
>> +u64 translate_cnthctl(u64 tcr);
>> +
>>  static inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
>>  {
>>  	return !(vcpu->arch.hcr_el2 & HCR_RW);
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 2d4290d2513a..dae9c42a7219 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -217,6 +217,11 @@ enum vcpu_sysreg {
>>  	NR_SYS_REGS	/* Nothing after this line! */
>>  };
>>  
>> +static inline bool sysreg_is_el2(int reg)
>> +{
>> +	return reg >= FIRST_EL2_SYSREG && reg < NR_SYS_REGS;
>> +}
>> +
>>  /* 32bit mapping */
>>  #define c0_MPIDR	(MPIDR_EL1 * 2)	/* MultiProcessor ID Register */
>>  #define c0_CSSELR	(CSSELR_EL1 * 2)/* Cache Size Selection Register */
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index 693dd063c9c2..d024114da162 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -76,11 +76,142 @@ static bool write_to_read_only(struct kvm_vcpu *vcpu,
>>  	return false;
>>  }
>>  
>> +static u64 tcr_el2_ips_to_tcr_el1_ps(u64 tcr_el2)
> The code seems to suggest that you are translating TCR_EL2.PS to TCR_EL1.IPS.
> Perhaps the function should be named tcr_el2_ps_to_tcr_el1_ips?

yup.

>> +{
>> +	return ((tcr_el2 & TCR_EL2_PS_MASK) >> TCR_EL2_PS_SHIFT)
>> +		<< TCR_IPS_SHIFT;
>> +}
>> +
>> +u64 translate_tcr(u64 tcr)
>> +{
>> +	return TCR_EPD1_MASK |				/* disable TTBR1_EL1 */
>> +	       ((tcr & TCR_EL2_TBI) ? TCR_TBI0 : 0) |
>> +	       tcr_el2_ips_to_tcr_el1_ps(tcr) |
>> +	       (tcr & TCR_EL2_TG0_MASK) |
>> +	       (tcr & TCR_EL2_ORGN0_MASK) |
>> +	       (tcr & TCR_EL2_IRGN0_MASK) |
>> +	       (tcr & TCR_EL2_T0SZ_MASK);
>> +}
>> +
>> +u64 translate_cptr(u64 cptr_el2)
> The argument name is not consistent with the other translate_* functions. I
> think it is reasonably obvious that you are translating an EL2 register.

That's pretty much immaterial, and the variable could be called zorglub.
Consistency is good, but I don't think we need to worry about that level
of detail.

>> +{
>> +	u64 cpacr_el1 = 0;
>> +
>> +	if (!(cptr_el2 & CPTR_EL2_TFP))
>> +		cpacr_el1 |= CPACR_EL1_FPEN;
>> +	if (cptr_el2 & CPTR_EL2_TTA)
>> +		cpacr_el1 |= CPACR_EL1_TTA;
>> +	if (!(cptr_el2 & CPTR_EL2_TZ))
>> +		cpacr_el1 |= CPACR_EL1_ZEN;
>> +
>> +	return cpacr_el1;
>> +}
>> +
>> +u64 translate_sctlr(u64 sctlr)
>> +{
>> +	/* Bit 20 is RES1 in SCTLR_EL1, but RES0 in SCTLR_EL2 */
>> +	return sctlr | BIT(20);
>> +}
>> +
>> +u64 translate_ttbr0(u64 ttbr0)
>> +{
>> +	/* Force ASID to 0 (ASID 0 or RES0) */
> Are you forcing ASID to 0 because you are only expecting a non-vhe guest
> hypervisor to access ttbr0_el2, in which case the architecture says that the
> ASID field is RES0? Is it so unlikely that a vhe guest hypervisor will access
> ttbr0_el2 directly that it's not worth adding a check for that?

Like all the translate_* function, this is only called when running a
non-VHE guest so that the EL2 register is translated to the EL1 format.
A VHE guest usually has its sysregs in the EL1 format, and certainly
does for TTBR0_EL2.

>> +	return ttbr0 & ~GENMASK_ULL(63, 48);
>> +}
>> +
>> +u64 translate_cnthctl(u64 cnthctl)
>> +{
>> +	return ((cnthctl & 0x3) << 10) | (cnthctl & 0xfc);
> 
> Patch 16/59 "KVM: arm64: nv: Save/Restore vEL2 sysregs" suggests that you are
> translating CNTHCTL to write it to CNTKCTL_EL1. Looking at ARM DDI 0487D.b,
> CNTKCTL_EL1 has bits 63:10 RES0. I think the correct value should be ((cnthctl &
> 0x3) << 8) | (cnthctl & 0xfc).

Rookie mistake! When HCR_EL2.E2h==1 (which is always the case for NV),
CNTKCTL_EL1 accesses CNTHCTL_EL2. What you have here is the translation
of non-VHE CNTHCTL_EL2 to its VHE equivalent.

> 
>> +}
>> +
>> +#define EL2_SYSREG(el2, el1, translate)	\
>> +	[el2 - FIRST_EL2_SYSREG] = { el2, el1, translate }
>> +#define PURE_EL2_SYSREG(el2) \
>> +	[el2 - FIRST_EL2_SYSREG] = { el2,__INVALID_SYSREG__, NULL }
>> +/*
>> + * Associate vEL2 registers to their EL1 counterparts on the CPU.
>> + * The translate function can be NULL, when the register layout is identical.
>> + */
>> +struct el2_sysreg_map {
>> +	int sysreg;	/* EL2 register index into the array above */
>> +	int mapping;	/* associated EL1 register */
>> +	u64 (*translate)(u64 value);
>> +} nested_sysreg_map[NR_SYS_REGS - FIRST_EL2_SYSREG] = {
>> +	PURE_EL2_SYSREG( VPIDR_EL2 ),
>> +	PURE_EL2_SYSREG( VMPIDR_EL2 ),
>> +	PURE_EL2_SYSREG( ACTLR_EL2 ),
>> +	PURE_EL2_SYSREG( HCR_EL2 ),
>> +	PURE_EL2_SYSREG( MDCR_EL2 ),
>> +	PURE_EL2_SYSREG( HSTR_EL2 ),
>> +	PURE_EL2_SYSREG( HACR_EL2 ),
>> +	PURE_EL2_SYSREG( VTTBR_EL2 ),
>> +	PURE_EL2_SYSREG( VTCR_EL2 ),
>> +	PURE_EL2_SYSREG( RVBAR_EL2 ),
>> +	PURE_EL2_SYSREG( RMR_EL2 ),
>> +	PURE_EL2_SYSREG( TPIDR_EL2 ),
>> +	PURE_EL2_SYSREG( CNTVOFF_EL2 ),
>> +	PURE_EL2_SYSREG( CNTHCTL_EL2 ),
>> +	PURE_EL2_SYSREG( HPFAR_EL2 ),
>> +	EL2_SYSREG(      SCTLR_EL2,  SCTLR_EL1,      translate_sctlr ),
>> +	EL2_SYSREG(      CPTR_EL2,   CPACR_EL1,      translate_cptr  ),
>> +	EL2_SYSREG(      TTBR0_EL2,  TTBR0_EL1,      translate_ttbr0 ),
>> +	EL2_SYSREG(      TTBR1_EL2,  TTBR1_EL1,      NULL            ),
>> +	EL2_SYSREG(      TCR_EL2,    TCR_EL1,        translate_tcr   ),
>> +	EL2_SYSREG(      VBAR_EL2,   VBAR_EL1,       NULL            ),
>> +	EL2_SYSREG(      AFSR0_EL2,  AFSR0_EL1,      NULL            ),
>> +	EL2_SYSREG(      AFSR1_EL2,  AFSR1_EL1,      NULL            ),
>> +	EL2_SYSREG(      ESR_EL2,    ESR_EL1,        NULL            ),
>> +	EL2_SYSREG(      FAR_EL2,    FAR_EL1,        NULL            ),
>> +	EL2_SYSREG(      MAIR_EL2,   MAIR_EL1,       NULL            ),
>> +	EL2_SYSREG(      AMAIR_EL2,  AMAIR_EL1,      NULL            ),
>> +};
> Figuring out which registers are in this map and which aren't and are supposed
> to be treated differently is really cumbersome because they are split into two
> types of el2 registers and their order is different from the order in enum
> vcpu_sysreg (in kvm_host.h). Perhaps adding a comment about what registers will
> be treated differently would make the code a bit easier to follow?

I'm not sure what this buys us. We have 3 categories of EL2 sysregs:
- Purely emulated
- Directly mapped onto an EL1 sysreg
- Translated from EL2 to EL1

I think the wrappers represent that pretty well, although we could split
EL2_SYSREG into DIRECT_EL2_SYSREG and TRANSLATE_EL2_SYSREG. As for the
order, does it really matter? We also have the trap table order, which
is also different from the enum. Do you propose we reorder everything?

>> +
>> +static
>> +const struct el2_sysreg_map *find_el2_sysreg(const struct el2_sysreg_map *map,
>> +					     int reg)
>> +{
>> +	const struct el2_sysreg_map *entry;
>> +
>> +	if (!sysreg_is_el2(reg))
>> +		return NULL;
>> +
>> +	entry = &nested_sysreg_map[reg - FIRST_EL2_SYSREG];
>> +	if (entry->sysreg == __INVALID_SYSREG__)
>> +		return NULL;
>> +
>> +	return entry;
>> +}
>> +
>>  u64 vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, int reg)
>>  {
>> +
>>  	if (!vcpu->arch.sysregs_loaded_on_cpu)
>>  		goto immediate_read;
>>  
>> +	if (unlikely(sysreg_is_el2(reg))) {
>> +		const struct el2_sysreg_map *el2_reg;
>> +
>> +		if (!is_hyp_ctxt(vcpu))
>> +			goto immediate_read;
> I'm confused by this. is_hyp_ctxt returns false when the guest is not in vEL2
> AND HCR_EL.E2H or HCR_EL2.TGE are not set. In this case, the NV bit will not be
> set and the hardware will raise an undefined instruction exception when
> accessing an EL2 register from EL1. What am I missing?

You don't necessarily access an EL2 register just because you run at
EL2. You also access it because you emulate an EL1 instruction whose
behaviour is conditioned by an EL2 register.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

_______________________________________________
linux-arm-kernel mailing list
[email-masked]
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel