Date:   Sat, 2 Mar 2019 16:39:12 +0530
From:   Amit Daniel Kachhap <amit.kachhap@....com>
To:     James Morse <james.morse@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Christoffer Dall <christoffer.dall@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Jones <drjones@...hat.com>,
        Dave Martin <Dave.Martin@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Kristina Martsenko <kristina.martsenko@....com>,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Julien Thierry <julien.thierry@....com>
Subject: Re: [PATCH v6 1/6] arm64/kvm: preserve host HCR_EL2 value

Hi,

On 2/25/19 11:09 PM, James Morse wrote:
> Hi Amit,
> 
> On 19/02/2019 09:24, Amit Daniel Kachhap wrote:
>> From: Mark Rutland <mark.rutland@....com>
>>
>> When restoring HCR_EL2 for the host, KVM uses HCR_HOST_VHE_FLAGS, which
>> is a constant value. This works today, as the host HCR_EL2 value is
>> always the same, but this will get in the way of supporting extensions
>> that require HCR_EL2 bits to be set conditionally for the host.
>>
>> To allow such features to work without KVM having to explicitly handle
>> every possible host feature combination, this patch has KVM save/restore
>> for the host HCR when switching to/from a guest HCR. The saving of the
>> register is done once during cpu hypervisor initialization state and is
>> just restored after switch from guest.
>>
>> For fetching HCR_EL2 during kvm initialisation, a hyp call is made using
>> kvm_call_hyp and is helpful in NHVE case.
>>
>> For the hyp TLB maintenance code, __tlb_switch_to_host_vhe() is updated
>> to toggle the TGE bit with a RMW sequence, as we already do in
>> __tlb_switch_to_guest_vhe().
>>
>> The value of hcr_el2 is now stored in struct kvm_cpu_context as both host
>> and guest can now use this field in a common way.
> 
> 
>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> index ca56537..05706b4 100644
>> --- a/arch/arm/include/asm/kvm_host.h
>> +++ b/arch/arm/include/asm/kvm_host.h
>> @@ -273,6 +273,8 @@ static inline void __cpu_init_stage2(void)
>>   	kvm_call_hyp(__init_stage2_translation);
>>   }
>>   
>> +static inline void __cpu_copy_hyp_conf(void) {}
>> +
> 
> I agree Mark's suggestion of adding 'host_ctxt' in here makes it clearer what it is.
ok.
> 
> 
>> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
>> index 506386a..0dbe795 100644
>> --- a/arch/arm64/include/asm/kvm_emulate.h
>> +++ b/arch/arm64/include/asm/kvm_emulate.h
> 
> Hmmm, there is still a fair amount of churn due to moving the struct definition, but its
> easy enough to ignore as its mechanical. A preparatory patch that switched as may as
> possible to '*vcpu_hcr() = ' would cut the churn down some more, but I don't think its
> worth the extra effort.
> 
> 
>> diff --git a/arch/arm64/include/asm/kvm_hyp.h b/arch/arm64/include/asm/kvm_hyp.h
>> index a80a7ef..6e65cad 100644
>> --- a/arch/arm64/include/asm/kvm_hyp.h
>> +++ b/arch/arm64/include/asm/kvm_hyp.h
>> @@ -151,7 +151,7 @@ void __fpsimd_restore_state(struct user_fpsimd_state *fp_regs);
>>   bool __fpsimd_enabled(void);
>>   
>>   void activate_traps_vhe_load(struct kvm_vcpu *vcpu);
>> -void deactivate_traps_vhe_put(void);
>> +void deactivate_traps_vhe_put(struct kvm_vcpu *vcpu);
> 
> I've forgotten why this is needed. You don't add a user of vcpu to
> deactivate_traps_vhe_put() in this patch.
> 
> 
>> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
>> index b0b1478..006bd33 100644
>> --- a/arch/arm64/kvm/hyp/switch.c
>> +++ b/arch/arm64/kvm/hyp/switch.c
>> @@ -191,7 +194,7 @@ void activate_traps_vhe_load(struct kvm_vcpu *vcpu)
> 
>> -void deactivate_traps_vhe_put(void)
>> +void deactivate_traps_vhe_put(struct kvm_vcpu *vcpu)
>>   {
>>   	u64 mdcr_el2 = read_sysreg(mdcr_el2);
>>   
> 
> Why does deactivate_traps_vhe_put() need the vcpu?
vcpu is needed for the next patch which saves/restore mdcr_el2. I will 
add this in that patch.
> 
> 
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 7732d0b..1b2e05b 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -458,6 +459,16 @@ int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>>
>>   static inline void __cpu_init_stage2(void) {}
>>
>> +/**
>> + * __cpu_copy_hyp_conf - copy the boot hyp configuration registers
>> + *
>> + * It is called once per-cpu during CPU hyp initialisation.
>> + */
> 
> Is it just the boot cpu?
> 
> 
>> +static inline void __cpu_copy_hyp_conf(void)
>> +{
>> +	kvm_call_hyp(__kvm_populate_host_regs);
>> +}
>> +
> 
> 
>> diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
>> index 68d6f7c..68ddc0f 100644
>> --- a/arch/arm64/kvm/hyp/sysreg-sr.c
>> +++ b/arch/arm64/kvm/hyp/sysreg-sr.c
>> @@ -21,6 +21,7 @@
>>   #include <asm/kvm_asm.h>
>>   #include <asm/kvm_emulate.h>
>>   #include <asm/kvm_hyp.h>
>> +#include <asm/kvm_mmu.h>
> 
> ... what's kvm_mmu.h needed for?
> The __hyp_this_cpu_ptr() you add comes from kvm_asm.h.
> 
> /me tries it.
> 
> Heh, hyp_symbol_addr(). kvm_asm.h should include this, but can't because the
> kvm_ksym_ref() dependency is the other-way round. This is just going to bite us somewhere
> else later!
> If we want to fix it now, moving hyp_symbol_addr() to kvm_asm.h would fix it. It's
> generating adrp/add so the 'asm' label is fair, and it really should live with its EL1
> counterpart kvm_ksym_ref().
> 
Yes moving hyp_symbol_addr() fixes the dependency error.
> 
>> @@ -294,7 +295,7 @@ void kvm_vcpu_put_sysregs(struct kvm_vcpu *vcpu)
>>   	if (!has_vhe())
>>   		return;
>>   
>> -	deactivate_traps_vhe_put();
>> +	deactivate_traps_vhe_put(vcpu);
>>   
>>   	__sysreg_save_el1_state(guest_ctxt);
>>   	__sysreg_save_user_state(guest_ctxt);
>> @@ -316,3 +317,21 @@ void __hyp_text __kvm_enable_ssbs(void)
>>   	"msr	sctlr_el2, %0"
>>   	: "=&r" (tmp) : "L" (SCTLR_ELx_DSSBS));
>>   }
>> +
>> +/**
>> + * __kvm_populate_host_regs - Stores host register values
>> + *
>> + * This function acts as a function handler parameter for kvm_call_hyp and
>> + * may be called from EL1 exception level to fetch the register value.
>> + */
>> +void __hyp_text __kvm_populate_host_regs(void)
>> +{
>> +	struct kvm_cpu_context *host_ctxt;
> 
> 
>> +	if (has_vhe())
>> +		host_ctxt = this_cpu_ptr(&kvm_host_cpu_state);
>> +	else
>> +		host_ctxt = __hyp_this_cpu_ptr(kvm_host_cpu_state);
> 
> You can use __hyp_this_cpu_ptr() here, even on VHE.
> 
> For VHE the guts are the same and its simpler to use the same version in both cases.
> 
> 
> __hyp_this_cpu_ptr(sym) == hyp_symbol_addr(sym) + tpidr_el2;
> 
> hyp_symbol_addr() here is just to guarantee the address is generated based on where we're
> executing from, not loaded from a literal pool which would give us the link-time address.
> (or whenever kaslr applied the relocations). This matters for non-VHE because the compiler
> can't know the code has an EL2 address as well as its link-time address.
> 
> This doesn't matter for VHE, as there is no additional different address.
> 
> (the other trickery is on non-VHE the tpidr_el2 value isn't actually the same as the
> hosts.. but on VHE it is)
> 
> 
Thanks for the details.

>> +	host_ctxt->hcr_el2 = read_sysreg(hcr_el2);
>> +}
> 
> 
>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>> index 9e350fd3..8e18f7f 100644
>> --- a/virt/kvm/arm/arm.c
>> +++ b/virt/kvm/arm/arm.c
>> @@ -1328,6 +1328,7 @@ static void cpu_hyp_reinit(void)
>>   		cpu_init_hyp_mode(NULL);
>>   
>>   	kvm_arm_init_debug();
>> +	__cpu_copy_hyp_conf();
> 
> Your commit message says:
> | The saving of the register is done once during cpu hypervisor initialization state
> 
> But cpu_hyp_reinit() is called each time secondary CPUs come online. Its also called as
> part of the cpu-idle mechanism via hyp_init_cpu_pm_notifier(). cpu-idle can ask the
> firmware to power-off the CPU until an interrupt becomes pending for it. KVM's EL2 state
> disappears when this happens, these calls take care of setting it back up again. On Juno,
> this can happen tens of times a second, and this adds an extra call to EL2.
> 
> init_subsystems() would be the alternative place for this, but it wouldn't catch CPUs that
> came online after booting. I think you need something in cpu_hyp_reinit() or
> __cpu_copy_hyp_conf() to ensure it only happens once per CPU.
ok i will check on it.
> 
> I think you can test whether the HCR_EL2 value is zero, assuming zero means uninitialised.
> A VHE system would always set E2H, and a non-VHE system has to set RW.
It is not zero and is set to initial values.

Thanks,
Amit D
> 
> 
>>   	if (vgic_present)
>>   		kvm_vgic_init_cpu_hardware();
>>
> 
> 
> Thanks,
> 
> James
> 

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