From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 859D07347D for ; Thu, 25 Jul 2024 21:04:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721941481; cv=none; b=JZElHqtadmvWYNLVi3FXAYUOtLiikHQzAlRzmtsywRvfElZC1SS3nzjwCxAGJW7ur908iiSAFAcKNj4AAhukvJHhl3+SujvgOTI04/HySJKhvj6hpboIeWpPNAb63NGcWwbTxIItnfT42BOPTNw7/0VQYrTI0BnvT6bW5CVZ6GQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721941481; c=relaxed/simple; bh=FEWVOD3+yRKcEuqLgqxmvL5aypghEhU0c65lOoNrXl0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tafTV6/OMh2IQmLgi29E+J2sEFM6+idHeBddpDx1LlXWu/gnlYHWH/0TWGANcE1WR8LT8cZynPal0y6v2nrRl94T6QrQSFKp+9nV/6foefSihV6PLQelw6rVbPTeaizQO0WCfUT1e0ccfs44vbJxc8udh00Ynay9xgrDtMaC8jA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Hz73QVdY; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Hz73QVdY" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1721941478; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=tTDtUQaLZ7CrF67TfvKh0JSNTMNAlAQ/IuXCIFvbTU8=; b=Hz73QVdYJSgFJ9U9vYr8Cia9eiPCuKUqcojCIQvi/nzkLmVEpEKaAb+EdNRcBNaRjzgVxX 5hYxly59IhviuwPBU/lAs9UKXJVtdajQHd8F8G2dB5rWR7kpYyc/QC+WS0G0s2+h3B2tOm 7eVqJguzNJgCVp3OYsFL9S64cb2L1C8= Received: from mail-wr1-f71.google.com (mail-wr1-f71.google.com [209.85.221.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-329-fNmj5R8-OrGMYc48YPmfuA-1; Thu, 25 Jul 2024 17:04:37 -0400 X-MC-Unique: fNmj5R8-OrGMYc48YPmfuA-1 Received: by mail-wr1-f71.google.com with SMTP id ffacd0b85a97d-36873a449dfso1224598f8f.0 for ; Thu, 25 Jul 2024 14:04:36 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1721941476; x=1722546276; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=tTDtUQaLZ7CrF67TfvKh0JSNTMNAlAQ/IuXCIFvbTU8=; b=vmYmyFz0BSS1SSm9U94Rw5mPRgNQee9/8dSsuGCd1TqTIL9EulZbZICZTTDlTE89ZK NvfAfas0pXu4wQzBlcKolnm6UNMutc5ZePK7QM/75kYaE/ag1nyyAutekTS3zGEN9xfx c5C+rZYdEO06UOiNOSAdRNdinY+iob3WVNnaX2777toE020x+btZ/UG83FQSn7G3q6z2 LvTuwT8BhpGQpATZk/pnWWwO8uG/OOmSNACh5LMbf4jHTDogkSOGlu1HfqqZP2Q0+kWS Dquw2OzHCGivXLmIIWJZJwgiGGp0H/18WkTYh63D1UTyKA1HV3FgqQcAw3V45HgPQdls 3WWg== X-Forwarded-Encrypted: i=1; AJvYcCUEfcD77kmicEeIE3a8HSefGHc3M+SPgsPE0H6XlH2KSeHWNh/J5GBYBTBAgvhvFcqRo6uUCSGEIolL+Vkkq18fHRDqqfJh X-Gm-Message-State: AOJu0Yy2P75FGh96+CegraP5Uz39gQ/9gvIFJ6KtDE6+OnCyEu/93gyw H1ar4h2gjtmQIzdoaDbDNYdO4iHw0FK9uNyc3XQI1cpj7+9Cj/uOvMSvHdv+ZFsdf20ehCq848b b6J0HeyfNjzfM+sFAOHDx9DYFIK87FSaUORBU2hu7TLwhrlZlIXHCUw== X-Received: by 2002:adf:ed90:0:b0:367:8fee:4434 with SMTP id ffacd0b85a97d-36b31b4d48dmr2687324f8f.16.1721941476018; Thu, 25 Jul 2024 14:04:36 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHhpTnkp42p1F3wTt6Fx20U3s9cQUoElQmJjW+LG0DsT7CR4O+1x/Cg4YZh3CZ7ii4p+45m0A== X-Received: by 2002:adf:ed90:0:b0:367:8fee:4434 with SMTP id ffacd0b85a97d-36b31b4d48dmr2687298f8f.16.1721941475033; Thu, 25 Jul 2024 14:04:35 -0700 (PDT) Received: from redhat.com ([2a02:14f:1f7:28ce:f21a:7e1e:6a9:f708]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36b36863d87sm3172908f8f.110.2024.07.25.14.04.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 25 Jul 2024 14:04:34 -0700 (PDT) Date: Thu, 25 Jul 2024 17:04:29 -0400 From: "Michael S. Tsirkin" To: David Woodhouse Cc: Richard Cochran , Peter Hilber , linux-kernel@vger.kernel.org, virtualization@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-rtc@vger.kernel.org, "Ridoux, Julien" , virtio-dev@lists.linux.dev, "Luu, Ryan" , "Chashper, David" , "Mohamed Abuelfotoh, Hazem" , "Christopher S . Hall" , Jason Wang , John Stultz , netdev@vger.kernel.org, Stephen Boyd , Thomas Gleixner , Xuan Zhuo , Marc Zyngier , Mark Rutland , Daniel Lezcano , Alessandro Zummo , Alexandre Belloni , qemu-devel , Simon Horman Subject: Re: [PATCH] ptp: Add vDSO-style vmclock support Message-ID: <20240725170328-mutt-send-email-mst@kernel.org> References: <20240725082828-mutt-send-email-mst@kernel.org> <20240725083215-mutt-send-email-mst@kernel.org> <98813a70f6d3377d3a9d502fd175be97334fcc87.camel@infradead.org> <20240725100351-mutt-send-email-mst@kernel.org> <2a27205bfc61e19355d360f428a98e2338ff68c3.camel@infradead.org> <20240725122603-mutt-send-email-mst@kernel.org> <0959390cad71b451dc19e5f9396d3f4fdb8fd46f.camel@infradead.org> <20240725163843-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Jul 25, 2024 at 10:00:24PM +0100, David Woodhouse wrote: > On Thu, 2024-07-25 at 16:50 -0400, Michael S. Tsirkin wrote: > > On Thu, Jul 25, 2024 at 08:35:40PM +0100, David Woodhouse wrote: > > > On Thu, 2024-07-25 at 12:38 -0400, Michael S. Tsirkin wrote: > > > > On Thu, Jul 25, 2024 at 04:18:43PM +0100, David Woodhouse wrote: > > > > > The use case isn't necessarily for all users of gettimeofday(), of > > > > > course; this is for those applications which *need* precision time. > > > > > Like distributed databases which rely on timestamps for coherency, and > > > > > users who get fined millions of dollars when LM messes up their clocks > > > > > and they put wrong timestamps on financial transactions. > > > > > > > > I would however worry that with all this pass through, > > > > applications have to be coded to each hypervisor or even > > > > version of the hypervisor. > > > > > > Yes, that would be a problem. Which is why I feel it's so important to > > > harmonise the contents of the shared memory, and I'm implementing it > > > both QEMU and $DAYJOB, as well as aligning with virtio-rtc. > > > > > > Writing an actual spec for this would be another thing that might help. > > > > > > I don't think the structure should be changing between hypervisors (and > > > especially versions). We *will* see a progression from simply providing > > > the disruption signal, to providing the full clock information so that > > > guests don't have to abort transactions while they resync their clock. > > > But that's perfectly fine. > > > > > > And it's also entirely agnostic to the mechanism by which the memory > > > region is *discovered*. It doesn't matter if it's ACPI, DT, a > > > hypervisor enlightenment, a BAR of a simple PCI device, virtio, or > > > anything else. > > > > > > ACPI is one of the *simplest* options for a hypervisor and guest to > > > implement, and doesn't prevent us from using the same structure in > > > virtio-rtc. I'm happy enough using ACPI and letting virtio-rtc come > > > along later. > > > > > > > virtio has been developed with the painful experience that we keep > > > > making mistakes, or coming up with new needed features, > > > > and that maintaining forward and backward compatibility > > > > becomes a whole lot harder than it seems in the beginning. > > > > > > Yes. But as you note, this shared memory structure is a userspace ABI > > > all of its own, so we get to make a completely *different* kind of > > > mistake :) > > > > > > > > > So, something I still don't completely understand. > > Can't the VDSO thing be written to by kernel? > > Let's say on LM, an interrupt triggers and kernel copies > > data from a specific device to the VDSO. > > > > Is that problematic somehow? I imagine there is a race where > > userspace reads vdso after lm but before kernel updated > > vdso - is that the concern? > > > > Then can't we fix it by interrupting all CPUs right after LM? > > > > To me that seems like a cleaner approach - we then compartmentalize > > the ABI issue - kernel has its own ABI against userspace, > > devices have their own ABI against kernel. > > It'd mean we need a way to detect that interrupt was sent, > > maybe yet another counter inside that structure. > > > > WDYT? > > > > By the way the same idea would work for snapshots - > > some people wanted to expose that info to userspace, too. > > > was there supposed to be text here, or did you just like this so much you decided to repost my mail ;) -- MST