From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f73.google.com (mail-pj1-f73.google.com [209.85.216.73]) (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 F126E610B for ; Mon, 29 Sep 2025 16:56:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759164963; cv=none; b=HI6eGGOSUYxNzk4NasH1GNt3KC7MQzGLd5r0UCLKdFXhu/CGF7v5rTkoNmAEdwsgRM+9+VgIDdlIkTFM/qTr1dLo601Fw3m1V5v1TZtxKuk2gQe2mvW6BRlXyrGPd17pH4K8QYQEf13EagZZjjczxJTRe2b08dQfxSUl2ACCidQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759164963; c=relaxed/simple; bh=aq3fWZR1dXZm7LJywJmxuW6r/1l6NXgn+AwCN9U5644=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=VqNZwo4Y2G51glMnc84pKak8kO2CS8N8k5Gd7lnlF9QDEFZcN2vAAwQqdbO5y4KgxKgMkCGiIYw+qIF9rYlfU834+cfynGHyXnKzkZ1lxVE4t5uj4kZV67YYlTxqEk3GQldp2tfvOMTjnGmyc6HErC/cy7UdbABg8B1FmO/DplM= 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=Nb1eBvOE; arc=none smtp.client-ip=209.85.216.73 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="Nb1eBvOE" Received: by mail-pj1-f73.google.com with SMTP id 98e67ed59e1d1-32ec69d22b2so4587916a91.1 for ; Mon, 29 Sep 2025 09:56:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1759164961; x=1759769761; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=9XbXmWXQ6yw7jgUXJGOGhYyHRPmszq8E1Tp17tZVzwo=; b=Nb1eBvOE6obtmOsCmHrBvHnCawNY9rIrIH/iH1NKD/aNY9viljgBfQRjYqEJlTtFEn UrtBM2RXIe3I7eF+NA2z7mDZf77VtTdjJUpFOPdyFQodPgf9XoI6nZIN8D2nccXURpwy 5aiDWfrDmm8wmmq0SjTXQqIv8b50uZLh3dHK/+2cswdL9mp7Kl8w4GGk1WOBPr/cMiKH xkOzjQp9g//jPnIj97D5CCYS/JtjpsIHIUqAzmUcUu4yr2GD/d8LI5LT7GHF37QzsMpI CtwZYNsIihyoK6OUmNh9JAEDUELZRPCodiWI+KNe+j9OBXQAU2SXpzTDxoemqzWg8QG8 efBw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1759164961; x=1759769761; h=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=9XbXmWXQ6yw7jgUXJGOGhYyHRPmszq8E1Tp17tZVzwo=; b=USd9ekzOi0dZMu7k95/EtV97VOyBDrfQSYJ/vK0xxJrRLFmv8C3aCD/D3DLHKwzFVS DkSQGY98LjmtxQW9NybWr84oBQC6yWShajI55FFoEAM8NQV8ExN7htXaaMcX3nMBLNc5 ieXKceO4QXdOJiCGCIJaW4pk8Dp7C7MdtAT1R+qLu3i9ziNkgjHQ734Jb57H2GT5F97A GMMeGeeI6bgn0BLDxa9ALOEC1QNtKt8WJ9GgWGsXyUM7dkd2wNwSloUxcqf6xblZrQrx vmuIc0M4pvnzXwE2pwD7gUGtWOW0R/RyjxqOeY534W8NOKA3hWFLfmSDzUh6uR5ACpKn kcwg== X-Forwarded-Encrypted: i=1; AJvYcCVQwZkI+u/ek0GkLM2dJ6pbyCEXuzJe4zjb8f/WJxaZlzqt2aqP+kyjetYNeB8IdpR3nkyqgyJiPYImmvE=@vger.kernel.org X-Gm-Message-State: AOJu0YyFy3gqMII56Eg+bulZs/9u1LEFoakkch8EM5r/RneK/tBPEc2z MxwUzgAGra3XEN+LKJo1KTg7SuN0sNbrz+0ZfQlgjdm+WAN9M8SbjDJL4wop9/K0Ef4YMRYsNv8 HB9toIA== X-Google-Smtp-Source: AGHT+IFLjIzTUpc45LEGOUkPN7hnf1kgAXdO/j3/rwcUcXXzok9aWCqi6sPp1tEcnfxySBXZMf749JBhUC8= X-Received: from pjst22.prod.google.com ([2002:a17:90b:196:b0:330:7be2:9bdc]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:3ece:b0:32b:6145:fa63 with SMTP id 98e67ed59e1d1-3342a216f73mr17472559a91.4.1759164961433; Mon, 29 Sep 2025 09:56:01 -0700 (PDT) Date: Mon, 29 Sep 2025 09:55:59 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250926163114.2626257-1-seanjc@google.com> <20250926163114.2626257-2-seanjc@google.com> Message-ID: Subject: Re: [PATCH 1/6] KVM: guest_memfd: Add DEFAULT_SHARED flag, reject user page faults if not set From: Sean Christopherson To: Ackerley Tng Cc: David Hildenbrand , Patrick Roy , Fuad Tabba , Paolo Bonzini , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Nikita Kalyazin , shivankg@amd.com Content-Type: text/plain; charset="us-ascii" On Mon, Sep 29, 2025, Ackerley Tng wrote: > David Hildenbrand writes: > > > GUEST_MEMFD_FLAG_DEFAULT_SHARED; > >>>> > >>>> At least for now, GUEST_MEMFD_FLAG_DEFAULT_SHARED and > >>>> GUEST_MEMFD_FLAG_MMAP don't make sense without each other. Is it worth > >>>> checking for that, at least until we have in-place conversion? Having > >>>> only GUEST_MEMFD_FLAG_DEFAULT_SHARED set, but GUEST_MEMFD_FLAG_MMAP, > >>>> isn't a useful combination. > >>>> > >>> > >>> I think it's okay to have the two flags be orthogonal from the start. > >> > >> I think I dimly remember someone at one of the guest_memfd syncs > >> bringing up a usecase for having a VMA even if all memory is private, > >> not for faulting anything in, but to do madvise or something? Maybe it > >> was the NUMA stuff? (+Shivank) > > > > Yes, that should be it. But we're never faulting in these pages, we only > > need the VMA (for the time being, until there is the in-place conversion). > > > > Yup, Sean's patch disables faulting if GUEST_MEMFD_FLAG_DEFAULT_SHARED > is not set, but mmap() is always enabled so madvise() still works. Hah! I totally intended that :-D > Requiring GUEST_MEMFD_FLAG_DEFAULT_SHARED to be set together with > GUEST_MEMFD_FLAG_MMAP would still allow madvise() to work since > GUEST_MEMFD_FLAG_DEFAULT_SHARED only gates faulting. > > To clarify, I'm still for making GUEST_MEMFD_FLAG_DEFAULT_SHARED > orthogonal to GUEST_MEMFD_FLAG_MMAP with no additional checks on top of > whatever's in this patch. :)