linux-kernel - Re: [PATCH v2 06/21] arm64: KVM: VHE: Patch out use of HVC

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Date:	Mon, 1 Feb 2016 17:28:32 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	KVM devel mailing list <kvm@...r.kernel.org>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 06/21] arm64: KVM: VHE: Patch out use of HVC

On 01/02/16 17:08, Ard Biesheuvel wrote:
> On 1 February 2016 at 17:20, Marc Zyngier <marc.zyngier@....com> wrote:
>> On 01/02/16 15:36, Catalin Marinas wrote:
>>> On Mon, Feb 01, 2016 at 01:34:16PM +0000, Marc Zyngier wrote:
>>>> On 01/02/16 13:16, Christoffer Dall wrote:
>>>>> On Mon, Jan 25, 2016 at 03:53:40PM +0000, Marc Zyngier wrote:
>>>>>> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
>>>>>> index 93e8d983..9e0683f 100644
>>>>>> --- a/arch/arm64/kvm/hyp/hyp-entry.S
>>>>>> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
>>>>>> @@ -38,6 +38,32 @@
>>>>>>    ldp     x0, x1, [sp], #16
>>>>>>  .endm
>>>>>>
>>>>>> +.macro do_el2_call
>>>>>> +  /*
>>>>>> +   * Shuffle the parameters before calling the function
>>>>>> +   * pointed to in x0. Assumes parameters in x[1,2,3].
>>>>>> +   */
>>>>>> +  stp     lr, xzr, [sp, #-16]!
>>>>>
>>>>> remind me why this pair isn't just doing "str" instead of "stp" with the
>>>>> xzr ?
>>>>
>>>> Because SP has to be aligned on a 16 bytes boundary at all times.
>>>
>>> You could do something like:
>>>
>>>       sub     sp, sp, #16
>>>       str     lr, [sp]
>>>
>>
>> Ah, fair enough. I'll fold that in.
>>
> 
> Since we're micro-reviewing: what's wrong with
> 
> str lr, [sp, #-16]!
> 
> ?
> 

I suspect that on most micro-architectures, a register writeback is
going to be slower than doing the sub independently.

I may be wrong, though.

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