From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marc Zyngier <marc.zyngier@arm.com>
Subject: Re: [PATCH v2 06/21] arm64: KVM: VHE: Patch out use of HVC
Date: Mon, 1 Feb 2016 17:28:32 +0000
Message-ID: <56AF95C0.3080006@arm.com>
References: <1453737235-16522-1-git-send-email-marc.zyngier@arm.com>
 <1453737235-16522-7-git-send-email-marc.zyngier@arm.com>
 <20160201131634.GI1478@cbox> <56AF5ED8.7010303@arm.com>
 <20160201153635.GH15514@e104818-lin.cambridge.arm.com>
 <56AF85D8.7080907@arm.com>
 <CAKv+Gu9_KG7Zss3g1OMzcnRKQFhCcYtbSDEk-gwwgqPP-R6DOA@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Cc: KVM devel mailing list <kvm@vger.kernel.org>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will.deacon@arm.com>,
 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>,
 "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Return-path: <kvmarm-bounces@lists.cs.columbia.edu>
In-Reply-To: <CAKv+Gu9_KG7Zss3g1OMzcnRKQFhCcYtbSDEk-gwwgqPP-R6DOA@mail.gmail.com>
List-Unsubscribe: <https://lists.cs.columbia.edu/mailman/options/kvmarm>,
 <mailto:kvmarm-request@lists.cs.columbia.edu?subject=unsubscribe>
List-Archive: <https://lists.cs.columbia.edu/pipermail/kvmarm>
List-Post: <mailto:kvmarm@lists.cs.columbia.edu>
List-Help: <mailto:kvmarm-request@lists.cs.columbia.edu?subject=help>
List-Subscribe: <https://lists.cs.columbia.edu/mailman/listinfo/kvmarm>,
 <mailto:kvmarm-request@lists.cs.columbia.edu?subject=subscribe>
Errors-To: kvmarm-bounces@lists.cs.columbia.edu
Sender: kvmarm-bounces@lists.cs.columbia.edu
List-Id: kvm.vger.kernel.org

On 01/02/16 17:08, Ard Biesheuvel wrote:
> On 1 February 2016 at 17:20, Marc Zyngier <marc.zyngier@arm.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...