From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f202.google.com (mail-pf1-f202.google.com [209.85.210.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 589E93876D3 for ; Wed, 7 Jan 2026 15:54:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.202 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767801306; cv=none; b=og+1tktZOLDCqk32t1MWjr2iRReFKwUtqXd6ivy9W1Hnuh710LPsopeg15jqrPRNBtxfV/GQvtNs2o+LHv0VJ30EXVCh+JmW8eWNPIP3aPNkTY40QAlNzIjK1Qhq83YYHnvmJqGL8Z0Z0VQ1uUbs3vjibo5GKlT3W77TdzfJbXw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767801306; c=relaxed/simple; bh=yXyephpPsv2jEvNRfKQKQirxxpGxIV/ri/oUYID04Lo=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=mcZ+7V00Av38nSpIeq+cuUvV/plTucal4/AOghrLN6h5e383PAT7xqccRV+YFsWQ8kx85QD7r5qxgpciATQc4u8mSMQGffkuZ2N8XgoQU7krvniCgblGPyG8+bU4XfzzwZS0YxE8O6YwhO+w98oY3R2Sc/TXavYwfU83cNGo0Ac= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=S8kAnC8p; arc=none smtp.client-ip=209.85.210.202 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="S8kAnC8p" Received: by mail-pf1-f202.google.com with SMTP id d2e1a72fcca58-7b952a966d7so4541781b3a.3 for ; Wed, 07 Jan 2026 07:54:58 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1767801295; x=1768406095; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:from:to:cc:subject:date:message-id :reply-to; bh=KGDxiUzmOiqUqvFGIUEpvgOPKZQcr6FM34ff6SZAbo8=; b=S8kAnC8ptdPgvVS0TAePJlKqiayWvABWD0EHRv4VpmejkVID7qdCBbjgaQ7OqJjk8x PN1lHi+XKn+UK18JbaWqpodcBxF39LbP++9qHXQD5EnTA4bM+CC0NNPiAS3Ed1nHFj9y euU1GiZlzc2O4w81+2v0B6Vfl56JwHMhrx6WeVVyQgD8irLYRWKKRNWLUeVNXZT+zVS0 4VCYtxKP3otQKmOws+lMhB/Z9aiaBB2ye3GXl8AcLzAXSz0DqDVRpEj0eTJryPpeNq2K 0wd5WmG/JAoV09mik412hikKlZ2Xsr+d2t7b/jBCNyWCYoRtwNtrA/v4CyWTocxQ/H8F peSA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1767801295; x=1768406095; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=KGDxiUzmOiqUqvFGIUEpvgOPKZQcr6FM34ff6SZAbo8=; b=tYnm8+vySyC7thfQJLjn8IHn6SKtNaEEyjamuMxFEUeFJrRYv+ZFjxGlDd+Gg01tkA ibv/5osUBSmK+LWBeVUxuvIy32E7IdChKodpqtsEEjhWrtCCWGbfibhbY7mWC7CpfQcU NXBVViQQMNE3wiJqpqiOFmB4LIsEdGjkUk6afPKUSgnGHKtGwne9LTmb2vVyx1shM6lP S+S5y/hhr5t2uAoPh4jpPe9lGuRwlUNsY+OPX5RsW4ZXkt4Wh24UWHOns4UrqI82Wh4h v2UXjJu/mVLTl9meyoY0ER0hIqi6mphn/XeBSMH8CVBzHg8AEPqOp6FX7MeZnFydmsJe sWuA== X-Forwarded-Encrypted: i=1; AJvYcCVp8lbEX6kNKJXpMtVg3wsAyPuzuVOxxAwcPe7Cl7x+4ZARHT/yhHlUMuoGHJ5UDtKUzcEU3avW9n2CvOI=@vger.kernel.org X-Gm-Message-State: AOJu0YwDTurNdqBHAWRzTgAVpBJNArUIyAmNS0E94C4HjaBycTdgmbGR DC6rRcj2Py2qHYDkidbplIhJJMZkjlLtLrdx8rNV45ntJqmig1JSi22m6iBqTpi8eYiikRiczPy 1IQXV7Q== X-Google-Smtp-Source: AGHT+IHqGFGS6LAIDF1DwPZvkOWcGq1Um8q0uNPgRXMhHrxgju2ZXystVmcBRs3crGdS6VopTdlgO395eMw= X-Received: from pfaf9.prod.google.com ([2002:a05:6a00:a119:b0:7dd:8bba:639d]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a00:4089:b0:7e8:43f5:bd28 with SMTP id d2e1a72fcca58-81b7fdc968fmr3065457b3a.61.1767801295223; Wed, 07 Jan 2026 07:54:55 -0800 (PST) Date: Wed, 7 Jan 2026 07:54:53 -0800 In-Reply-To: <20260105074814.GA10215@k08j02272.eu95sqa> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20231107202002.667900-1-aghulati@google.com> <20260105074814.GA10215@k08j02272.eu95sqa> Message-ID: Subject: Re: [RFC PATCH 00/14] Support multiple KVM modules on the same host From: Sean Christopherson To: Hou Wenlong Cc: Lai Jiangshan , Anish Ghulati , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, hpa@zytor.com, Vitaly Kuznetsov , peterz@infradead.org, paulmck@kernel.org, Mark Rutland Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable On Mon, Jan 05, 2026, Hou Wenlong wrote: > Sorry for revisiting this topic after a long time. I haven't seen any > new updates regarding this topic/series, and I didn=E2=80=99t find any re= cent > activity on the GitHub repository. Is the multi-KVM topic still being > considered for upstreaming, or is there anything blocking this? We have abandoned upstreaming multi-KVM. The operational cost+complexity i= s too high relative to the benefits, especially when factoring in things like ASI= and live patching, and the benefits are almost entirely obsoleted by kernel liv= e update support. > As Lai pointed out, we also have a similar multi-KVM implementation in > our internal environment, so we are quite interested in this topic. > Recently, when we upgraded our kernel version, we found that maintaining > multi-KVM has become a significant burden. Yeah, I can imagine the pain all too well. :-/ > We are willing to move forward with it if multi-KVM is still accepted for > upstream. So I look forward to feedback from the maintainers. > > From what I've seen, the recent patch set that enables VMX/SVM during > booting is a good starting point for multi-KVM as well. I have mixed feelings on multi-KVM. Without considering maintenance and su= pport costs, I still love the idea of reworking the kernel to support running mul= tiple hypervisors concurrently. But as I explained in the first cover letter of = that series[0], there is a massive amount of complexity, both in initial develop= ment and ongoing maintenance, needed to provide such infrastructure: : I got quite far long on rebasing some internal patches we have to extrac= t the : core virtualization bits out of KVM x86, but as I paged back in all of t= he : things we had punted on (because they were waaay out of scope for our ne= eds), : I realized more and more that providing truly generic virtualization : instrastructure is vastly different than providing infrastructure that c= an be : shared by multiple instances of KVM (or things very similar to KVM)[1]. : : So while I still don't want to blindly do VMXON, I also think that tryin= g to : actually support another in-tree hypervisor, without an imminent user to= drive : the development, is a waste of resources, and would saddle KVM with a pi= le of : pointless complexity. For deployment to a relatively homogeneous fleet, many of the pain points c= an be avoided by either avoiding them entirely or making the settings "inflexible= ", because there is effectively one use case and so such caveats are a non-iss= ue. But those types of simplifications don't work upstream, e.g. saying "eVMCS = is unsupported if multi-KVM is possible" instead of moving eVMCS enabling to a= base module isn't acceptable. So I guess my "official" stance is that I'm not opposed to upstreaming mult= i-KVM (or similar) functionality, but I'm not exactly in favor of it either. And practically speaking, because multi-KVM would be in constant conflict with = so much ongoing/new feature support (both in software and hardware), and is no= t a priority for anyone pursuing kernel live update, upstreaming would be likel= y take several years, without any guarantee of a successful landing. [0] https://lore.kernel.org/all/20251010220403.987927-1-seanjc@google.com [1] https://lore.kernel.org/all/aOl5EutrdL_OlVOO@google.com