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 E5B0622C9F7 for ; Thu, 16 Jan 2025 20:32:15 +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=1737059537; cv=none; b=Op8orwJ/dVxwL8/vU3rUM+3loYYKsxzS0o+ixGgLt1TT3RH1c5Vq+vfB+q3IBRZQtOFdTSWRITehSn7GWOTAPg6paEpU9/65v/vcaZ9YTyJ6ZcvACA9JtqJvE0Z/8HrOHI6BBdsT09t8rXDdkt9a8l+POhFhkVOrzEj2X6DgiTU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737059537; c=relaxed/simple; bh=Gp2dMCkhOZWquwQcvnyVZRJDHqMIikWYXh/aHRG4fJI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cvULEbOw3n04hvK0vqkG2j0R5ty1ub2xBNgMxXAH168TLJ++bbflVgZs/DtSGN/hkIojbkGnGYQzioI/BDb4mPQQyVV78HBIwuvPPNZ/DjDlTD6LcFUU7cGiz77HQ5hdTQRZW/wRGe2msdZaqsx/d0Cpj99FBkwnThxrM3odJ/I= 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=ABcboBLb; 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="ABcboBLb" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1737059534; 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=cBjXp+Q0kBMQo93pn9/JG/mDdpOMWtXcKEJJB7g0DX0=; b=ABcboBLbXspaBdHa60KRlbROCiAmMg0WeHLc5O78iv+xM3G284e953YOyGR9bKJ7cjMaYf ZYkQoagGCcFIsa3c4pZhS6F9IqVEiS+XOya8N7+vgHjz/GiWftBI/ZXSw6j9WQ1yhdpeGW DorEfLE8qyzxMVFbH08b7AqGNGgVO6Y= Received: from mail-pl1-f198.google.com (mail-pl1-f198.google.com [209.85.214.198]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-641-BAXTSCIBNaehm_LOGchvhQ-1; Thu, 16 Jan 2025 15:32:13 -0500 X-MC-Unique: BAXTSCIBNaehm_LOGchvhQ-1 X-Mimecast-MFC-AGG-ID: BAXTSCIBNaehm_LOGchvhQ Received: by mail-pl1-f198.google.com with SMTP id d9443c01a7336-2163d9a730aso30667875ad.1 for ; Thu, 16 Jan 2025 12:32:13 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1737059532; x=1737664332; 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=cBjXp+Q0kBMQo93pn9/JG/mDdpOMWtXcKEJJB7g0DX0=; b=eO+z8EcnN04dcaehtE6lJ2/sZ+TLJUz27LdycKw64wI7OHxxMdsvJ1S7RbuX6Kzaqb KOem8pwnwAwVHDcsCZAfFu/q6HUoLrGJ1jhFXaRpX9k5rTWBOzLXuPM2V6/L/YPKkbnM 1SCqTLw4F5PYt14fNqazhfmROakVy5JBmOls7RF8pQZVORYByGDQvVwOryoZs+p00uR4 +cnHV6mAtkqak+QFQBP94kmz52GjUOaZrkq7h6I9EDr25LyxHn/ySB6r6ms73wvbJD5O +qb55DsziMY7bozNOCh8FbaYRB4ID/flGfN4JQ0U58xZh6XIFgIdGVzk8bJ4qJVZ9PoC upHA== X-Forwarded-Encrypted: i=1; AJvYcCXxrtG1rjqYsSeDjTiIyA+KadBkxxEetTY5KecW3EEHiC6c4uSgpxl52qQPOqgDgb6VKR8=@vger.kernel.org X-Gm-Message-State: AOJu0YzUOwplMtbrZn5tlJLMxRFffs4JegkGJPfVaQPnF+9RC0U0PydG Z7/WisliNdOxfP0vHZ0wEQZyF4gPUtIvq+193X0xxRDF9rppHYVmCcgoXHbVEwPkvjfZmEYEg1j BZtFfPdYtwLlCxTpIPq3CGmFhsEpbXusRpw+gRZdi72AeEBNW5w== X-Gm-Gg: ASbGncuajhUxjbtp0wUASO94Qd82fGwCudUQK0S9W6yDn+co0wumcrCVdb72D4w/NHO wj5753nhQVI4nNf9a5kJhaE5NFVDLt5wyhV8Gt2ghSdnbxM/A+hmHmCiat5isX3rSaOLiadDOHy //w5r0mojabiykrmMiXv8GtIyHcCVpH74Ku0NYw5I4n1mw64E7xSWTFla2BwUkJx1ei6s+958zn gFUVSha9AhQgnwEGx6vqHqe7YS7kEATstgdyq34ZNm40WDkgCvCbiAhWbfQQy2GZ6Xd2UHiDhtW DQQMOwJA05NMg6YD9w== X-Received: by 2002:a17:903:238e:b0:216:311e:b1c4 with SMTP id d9443c01a7336-21bf029e049mr110709345ad.4.1737059532479; Thu, 16 Jan 2025 12:32:12 -0800 (PST) X-Google-Smtp-Source: AGHT+IEKxzy3YKG0/4peNFIGTyI8M6fxHbUDzV/T0Dy6Fv99Nr5xuebiMcqfPD2o4c5t4sCUc/gI8A== X-Received: by 2002:a17:903:238e:b0:216:311e:b1c4 with SMTP id d9443c01a7336-21bf029e049mr110708925ad.4.1737059532113; Thu, 16 Jan 2025 12:32:12 -0800 (PST) Received: from x1n (pool-99-254-114-190.cpe.net.cable.rogers.com. [99.254.114.190]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-a9bdd30e9adsm444712a12.62.2025.01.16.12.32.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Jan 2025 12:32:11 -0800 (PST) Date: Thu, 16 Jan 2025 15:32:06 -0500 From: Peter Xu To: James Houghton Cc: Paolo Bonzini , Sean Christopherson , Jonathan Corbet , Marc Zyngier , Oliver Upton , Yan Zhao , Nikita Kalyazin , Anish Moorthy , Peter Gonda , David Matlack , Wei W , kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev Subject: Re: [PATCH v1 00/13] KVM: Introduce KVM Userfault Message-ID: References: <20241204191349.1730936-1-jthoughton@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On Thu, Jan 16, 2025 at 03:19:49PM -0500, Peter Xu wrote: > James, > > Sorry for a late reply. > > I still do have one or two pure questions, but nothing directly relevant to > your series. > > On Thu, Jan 02, 2025 at 12:53:11PM -0500, James Houghton wrote: > > So I'm not pushing for KVM Userfault to replace userfaultfd; it's not > > worth the extra/duplicated complexity. And at LPC, Paolo and Sean > > indicated that this direction was indeed wrong. I have another way to > > make this work in mind. :) > > Do you still want to share it, more or less? :) > > > > > For the gmem case, userfaultfd cannot be used, so KVM Userfault isn't > > replacing it. And as of right now anyway, KVM Userfault *does* provide > > a complete post-copy system for gmem. > > > > When gmem pages can be mapped into userspace, for post-copy to remain > > functional, userspace-mapped gmem will need userfaultfd integration. > > Keep in mind that even after this integration happens, userfaultfd > > alone will *not* be a complete post-copy solution, as vCPU faults > > won't be resolved via the userspace page tables. > > Do you know in context of CoCo, whether a private page can be accessed at > all outside of KVM? > > I think I'm pretty sure now a private page can never be mapped to > userspace. However, can another module like vhost-kernel access it during > postcopy? My impression of that is still a yes, but then how about > vhost-user? > > Here, the "vhost-kernel" part represents a question on whether private > pages can be accessed at all outside KVM. While "vhost-user" part > represents a question on whether, if the previous vhost-kernel question > answers as "yes it can", such access attempt can happen in another > process/task (hence, not only does it lack KVM context, but also not > sharing the same task context). Right after I sent it, I just recalled whenever a device needs to access the page, it needs to be converted to shared pages first.. So I suppose the questions were not valid at all! It is not about the context but that the pages will be shared always whenever a device in whatever form will access it.. Fundamentally I'm thinking about whether userfaultfd must support (fd, offset) tuple. Now I suppose it's not, because vCPUs accessing private/shared will all exit to userspace, while all non-vCPU / devices can access shared pages only. In that case, looks like userfaultfd can support CoCo on device emulations by sticking with virtual-address traps like before, at least from that specific POV. -- Peter Xu