[PATCH 10/37] KVM: arm64: Slightly improve debug save/restore functions

[Julien Thierry]

Hi Christopher,

On 12/10/17 11:41, Christoffer Dall wrote:
> The debug save/restore functions can be improved by using the has_vhe()
> static key instead of the instruction alternative.  Using the static key
> uses the same paradigm as we're going to use elsewhere, it makes the
> code more readable, and it generates slightly better code (no
> stack setups and function calls unless necessary).
> 
> We also use a static key on the restore path, because it will be
> marginally faster than loading a value from memory.
> 
> Finally, we don't have to conditionally clear the debug dirty flag if
> it's set, we can just clear it.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
> ---
>   arch/arm64/kvm/hyp/debug-sr.c | 22 +++++++++-------------
>   1 file changed, 9 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
> index 0fc0758..a2291b6 100644
> --- a/arch/arm64/kvm/hyp/debug-sr.c
> +++ b/arch/arm64/kvm/hyp/debug-sr.c
> @@ -75,11 +75,6 @@
>   
>   #define psb_csync()		asm volatile("hint #17")
>   
> -static void __hyp_text __debug_save_spe_vhe(u64 *pmscr_el1)
> -{
> -	/* The vcpu can run. but it can't hide. */
> -}
> -
>   static void __hyp_text __debug_save_spe_nvhe(u64 *pmscr_el1)
>   {
>   	u64 reg;
> @@ -109,10 +104,6 @@ static void __hyp_text __debug_save_spe_nvhe(u64 *pmscr_el1)
>   	dsb(nsh);
>   }
>   
> -static hyp_alternate_select(__debug_save_spe,
> -			    __debug_save_spe_nvhe, __debug_save_spe_vhe,
> -			    ARM64_HAS_VIRT_HOST_EXTN);
> -
>   static void __hyp_text __debug_restore_spe(u64 pmscr_el1)
>   {
>   	if (!pmscr_el1)
> @@ -174,17 +165,22 @@ void __hyp_text __debug_cond_save_host_state(struct kvm_vcpu *vcpu)
>   {
>   	__debug_save_state(vcpu, &vcpu->arch.host_debug_state.regs,
>   			   kern_hyp_va(vcpu->arch.host_cpu_context));
> -	__debug_save_spe()(&vcpu->arch.host_debug_state.pmscr_el1);
> +
> +	/* Non-VHE: Disable and flush SPE data generation
> +	 * VHE: The vcpu can run. but it can't hide. */
> +	if (!has_vhe())
> +		__debug_save_spe_nvhe(&vcpu->arch.host_debug_state.pmscr_el1);
>   }
>   
>   void __hyp_text __debug_cond_restore_host_state(struct kvm_vcpu *vcpu)
>   {
> -	__debug_restore_spe(vcpu->arch.host_debug_state.pmscr_el1);
> +	if (!has_vhe())
> +		__debug_restore_spe(vcpu->arch.host_debug_state.pmscr_el1);

For consistency, would it be worth naming that function 
'__debug_restore_spe_nvhe' ?

Also, looking at __debug_save_spe_nvhe, I'm not sure how we guarantee 
that we might not end up using stale data during the restore_spe 
(though, if this is an issue, it existed before this change).
The save function might exit without setting a value to saved pmscr_el1.

Basically I'm wondering if the following scenario (in non VHE) is 
possible and/or whether it is problematic:

- save spe
- restore spe
- host starts using spi -> !(PMBLIMITR_EL1 & PMBLIMITR_EL1_E)
- save spe -> returns early without setting pmscr_el1
- restore spe with old save instead of doing nothing


Cheers,

-- 
Julien Thierry