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

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Date:   Mon, 11 Sep 2017 19:48:45 +0800
From:   gengdongjiu <gengdongjiu@...wei.com>
To:     Vladimir Murzin <vladimir.murzin@....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

Hi Vladimir,

On 2017/9/11 19:20, Vladimir Murzin wrote:
> 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...
thanks for the explanation.


> 
> 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...
I do not know when you are busy and in merge window


> 
> Cheers
> Vladimir
> 
> .
>