Subject: Re: [RFC PATCH 18/26] kvm: arm64: Intercept PSCI_CPU_OFF host SMC calls

From: Marc Zyngier <[email-masked]>
Date: Thu, 05 Nov 2020 11:30:44 +0000

On 2020-11-04 18:36, David Brazdil wrote:
> Add a handler of the CPU_OFF PSCI host SMC trapped in KVM nVHE hyp 
> code.
> When invoked, it changes the recorded state of the core to OFF before
> forwarding the call to EL3. If the call fails, it changes the state 
> back
> to ON and returns the error to the host.
> 
> Signed-off-by: David Brazdil <[email-masked]>
> ---
>  arch/arm64/kvm/hyp/nvhe/psci.c | 30 +++++++++++++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/psci.c 
> b/arch/arm64/kvm/hyp/nvhe/psci.c
> index c3d0a6246c66..00dc0cab860c 100644
> --- a/arch/arm64/kvm/hyp/nvhe/psci.c
> +++ b/arch/arm64/kvm/hyp/nvhe/psci.c
> @@ -13,6 +13,8 @@
>  #include <kvm/arm_psci.h>
>  #include <uapi/linux/psci.h>
> 
> +#include <nvhe/spinlock.h>
> +
>  /* Config options set by the host. */
>  u32 kvm_host_psci_version = PSCI_VERSION(0, 0);
>  u32 kvm_host_psci_function_id[PSCI_FN_MAX];
> @@ -20,6 +22,7 @@ s64 hyp_physvirt_offset;
> 
>  #define __hyp_pa(x) ((phys_addr_t)(x) + hyp_physvirt_offset)
> 
> +static DEFINE_PER_CPU(hyp_spinlock_t, psci_cpu_lock);
>  DEFINE_PER_CPU(enum kvm_nvhe_psci_state, psci_cpu_state);
> 
>  static u64 get_psci_func_id(struct kvm_cpu_context *host_ctxt)
> @@ -76,9 +79,32 @@ static __noreturn unsigned long
> psci_forward_noreturn(struct kvm_cpu_context *ho
>  	hyp_panic(); /* unreachable */
>  }
> 
> +static int psci_cpu_off(u64 func_id, struct kvm_cpu_context 
> *host_ctxt)
> +{
> +	hyp_spinlock_t *cpu_lock = this_cpu_ptr(&psci_cpu_lock);
> +	enum kvm_nvhe_psci_state *cpu_power = this_cpu_ptr(&psci_cpu_state);
> +	u32 power_state = (u32)host_ctxt->regs.regs[1];
> +	int ret;
> +
> +	/* Change the recorded state to OFF before forwarding the call. */
> +	hyp_spin_lock(cpu_lock);
> +	*cpu_power = KVM_NVHE_PSCI_CPU_OFF;
> +	hyp_spin_unlock(cpu_lock);

So at this point, another CPU can observe the vcpu being "off", and 
issue
a PSCI_ON, which may result in an "already on". I'm not sure this is an
actual issue, but it is worth documenting.

What is definitely missing is a rational about *why* we need to track 
the
state of the vcpus. I naively imagined that we could directly proxy the
PSCI calls to EL3, only repainting PC for PSCI_ON.

Thanks,

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

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