linux-kernel - Re: [PATCH] arm64: KVM: Skip PSTATE.PAN reest at EL2 in non-VHE

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Date:   Mon, 11 Sep 2017 12:20:50 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Dongjiu Geng <gengdongjiu@...wei.com>, christoffer.dall@...aro.org,
        marc.zyngier@....com, james.morse@....com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: KVM: Skip PSTATE.PAN reest at EL2 in non-VHE

On 11/09/17 12:16, Dongjiu Geng wrote:
> PSTATE.PAN disables reading and/or writing to a userspace virtual
> address from EL1 in non-VHE or from EL2 in VHE. In non-VHE, there is
> no any userspace mapping at EL2, so no need to reest the PSTATE.PAN.
                                                 ^^^^^
                                                 reset
> 
> Signed-off-by: Dongjiu Geng <gengdongjiu@...wei.com>
> Signed-off-by: Haibin Zhang <zhanghaibin7@...wei.com>
> ---
>  arch/arm64/kvm/hyp/entry.S | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index 12ee62d6d410..86a7549b1b4c 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -96,8 +96,12 @@ ENTRY(__guest_exit)
>  
>  	add	x1, x1, #VCPU_CONTEXT
>  
> -	ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN)
> +alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
> +	b	2f			// skip PAN at EL2 in non-VHE
> +alternative_else_nop_endif
>  
> +	ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN)
> +2:
>  	// Store the guest regs x2 and x3
>  	stp	x2, x3,   [x1, #CPU_XREG_OFFSET(2)]
>  
> 

Ok. Probably I need to say why original patch did not consider non-VHE case:
- VHE and PAN features come within the same v8.1 extension bundle, so it is
  unlucky to see IRL implementation with PAN but no VHE.
- Given above the only case where extra PAN instruction could count is
  VHE-enabled system with CONFIG_ARM64_VHE is not set; However, IMO, usecase for
  such setup is kind of debugging; it is quite obvious that those who care of
  performance should not disable VHE in the first place...

Nit:
In general it is not polite to keep posting patches in a middle of the merge
window - people are busy with more important stuff...

Cheers
Vladimir