From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marc Zyngier Subject: Re: [PATCH v2 00/21] arm64: KVM: world switch in C Date: Tue, 01 Dec 2015 09:58:23 +0000 Message-ID: <565D6F3F.5030700@arm.com> References: <1448650215-15218-1-git-send-email-marc.zyngier@arm.com> <20151130203345.GI11704@cbox> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, Ard Biesheuvel , Catalin Marinas , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org To: Christoffer Dall Return-path: In-Reply-To: <20151130203345.GI11704@cbox> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu List-Id: kvm.vger.kernel.org On 30/11/15 20:33, Christoffer Dall wrote: > On Fri, Nov 27, 2015 at 06:49:54PM +0000, Marc Zyngier wrote: >> Once upon a time, the KVM/arm64 world switch was a nice, clean, lean >> and mean piece of hand-crafted assembly code. Over time, features have >> crept in, the code has become harder to maintain, and the smallest >> change is a pain to introduce. The VHE patches are a prime example of >> why this doesn't work anymore. >> >> This series rewrites most of the existing assembly code in C, but keeps >> the existing code structure in place (most function names will look >> familiar to the reader). The biggest change is that we don't have to >> deal with a static register allocation (the compiler does it for us), >> we can easily follow structure and pointers, and only the lowest level >> is still in assembly code. Oh, and a negative diffstat. >> >> There is still a healthy dose of inline assembly (system register >> accessors, runtime code patching), but I've tried not to make it too >> invasive. The generated code, while not exactly brilliant, doesn't >> look too shaby. I do expect a small performance degradation, but I >> believe this is something we can improve over time (my initial >> measurements don't show any obvious regression though). > > I ran this through my experimental setup on m400 and got this: [...] > What this tells me is that we do take a noticable hit on the > world-switch path, which shows up in the TCP_RR and hackbench workloads, > which have a high precision in their output. > > Note that the memcached number is well within its variability between > individual benchmark runs, where it varies to 12% of its average in over > 80% of the executions. > > I don't think this is a showstopper thought, but we could consider > looking more closely at a breakdown of the world-switch path and verify > if/where we are really taking a hit. Thanks for doing so, very interesting. As a data point, what compiler are you using? I'd expect some variability based on the compiler version... Thanks, M. -- Jazz is not dead. It just smells funny...