Re: [PATCH v6 1/6] arm64/kvm: preserve host HCR_EL2 value

[Amit Daniel Kachhap <amit.kachhap@arm.com>]

Hi,

On 2/21/19 9:19 PM, Dave Martin wrote:
> On Tue, Feb 19, 2019 at 02:54:26PM +0530, Amit Daniel Kachhap wrote:
>> From: Mark Rutland <mark.rutland@arm.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.
> 
> Minor nit: NVHE misspelled.  This looks a bit like it's naming an arch
> feature rather than a kernel implementation detail though.  Maybe write
> "non-VHE".
yes.
> 
>> 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.
>>
>> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
>> [Added __cpu_copy_hyp_conf, hcr_el2 field in struct kvm_cpu_context]
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>> Cc: kvmarm@lists.cs.columbia.edu
>> ---
>>   arch/arm/include/asm/kvm_host.h      |  2 ++
>>   arch/arm64/include/asm/kvm_asm.h     |  2 ++
>>   arch/arm64/include/asm/kvm_emulate.h | 22 +++++++++++-----------
>>   arch/arm64/include/asm/kvm_host.h    | 13 ++++++++++++-
>>   arch/arm64/include/asm/kvm_hyp.h     |  2 +-
>>   arch/arm64/kvm/guest.c               |  2 +-
>>   arch/arm64/kvm/hyp/switch.c          | 23 +++++++++++++----------
>>   arch/arm64/kvm/hyp/sysreg-sr.c       | 21 ++++++++++++++++++++-
>>   arch/arm64/kvm/hyp/tlb.c             |  6 +++++-
>>   virt/kvm/arm/arm.c                   |  1 +
>>   10 files changed, 68 insertions(+), 26 deletions(-)
>>
>> 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) {}
>> +
>>   static inline int kvm_arch_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>>   {
>>   	return 0;
>> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
>> index f5b79e9..8acd73f 100644
>> --- a/arch/arm64/include/asm/kvm_asm.h
>> +++ b/arch/arm64/include/asm/kvm_asm.h
>> @@ -80,6 +80,8 @@ extern void __vgic_v3_init_lrs(void);
>>   
>>   extern u32 __kvm_get_mdcr_el2(void);
>>   
>> +extern void __kvm_populate_host_regs(void);
>> +
>>   /* Home-grown __this_cpu_{ptr,read} variants that always work at HYP */
>>   #define __hyp_this_cpu_ptr(sym)						\
>>   	({								\
>> 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
>> @@ -50,25 +50,25 @@ void kvm_inject_pabt32(struct kvm_vcpu *vcpu, unsigned long addr);
>>   
>>   static inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
>>   {
>> -	return !(vcpu->arch.hcr_el2 & HCR_RW);
>> +	return !(vcpu->arch.ctxt.hcr_el2 & HCR_RW);
> 
> Putting hcr_el2 into struct kvm_cpu_context creates a lot of splatter
> here, and I'm wondering whether it's really necessary.  Otherwise,
> we could just put the per-vcpu guest HCR_EL2 value in struct
> kvm_vcpu_arch.
I did like that in V4 version [1] but comments were raised that this was 
repetition of hcr_el2 field in 2 places and may be avoided.

[1]: https://lkml.org/lkml/2019/1/4/433
> 
> Is the *host* hcr_el2 value really different per-vcpu?  That looks
> odd.  I would have thought this is fixed across the system at KVM
> startup time.
> 
> Having a single global host hcr_el2 would also avoid the need for
> __kvm_populate_host_regs(): instead, we just decide what HCR_EL2 is to
> be ahead of time and set a global variable that we map into Hyp.
> 
> 
> Or does the host HCR_EL2 need to vary at runtime for some reason I've
> missed?
This patch basically makes host hcr_el2 not to use fixed values like 
HCR_HOST_NVHE_FLAGS/HCR_HOST_VHE_FLAGS during context switch and hence 
saves those values at boot time. This patch is just preparation to 
configure host hcr_el2 dynamically. However currently it is same for all 
cpus.

I suppose it is better to have host hcr_el2 as percpu to take care of 
heterogeneous systems. Currently even host mdcr_el2 is stored on percpu 
basis(arch/arm64/kvm/debug.c).
> 
> [...]
> 
> +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);
> 
> According to the comment by the definition of __hyp_this_cpu_ptr(), this
> always works at Hyp.  I also see other calls with no fallback
> this_cpu_ptr() call like we have here.
> 
> So, can we simply always call __hyp_this_cpu_ptr() here?
Yes i missed this.

Thanks,
Amit D
> 
> (I'm not familiar with this, myself.)
> 
> Cheers
> ---Dave
> 

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