From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 9510AC4332F for ; Thu, 2 Nov 2023 16:04:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:Cc:To:From:Subject:Message-ID: References:Mime-Version:In-Reply-To:Date:Reply-To:Content-ID: Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Owner; bh=ay8rPfYsJ2d7DEV6Paye4AZYdzGs8Xo1rBDYDFVzW4Q=; b=I1Q1mRk1AzwuWVYUJZ5b0rF4ZK tRBLAZcj62Qaot4wAOXWkUANW/KmP834FYfR7IS1k+xyMP8D1L9mWsNjud5t/nB2JCQieyrr1211a /j9v/Bh3c2Ypi4AXgh8whnmcwBnv6AQ2nvJmKxl4LgtvGpw3uoSRzlSy1Y4WCrUzzDnuwJ0nIDrqD fzsfupxxuqfaRWBrr8k8oKNe2AgDebR2zaX6IGqigFIsmXWRwZl525gxih6JoZhIrV5OW5WcG1Ngf TJnueSzyxKf6FQQR7V2warEj5sZnyTayAWaC2vgK90OUqjwhcKR+jcCqbesaeOjNr1iZRLNAYNyeQ KOo/RAuQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1qyaAg-009pby-0R; Thu, 02 Nov 2023 16:03:50 +0000 Received: from mail-pl1-x649.google.com ([2607:f8b0:4864:20::649]) by bombadil.infradead.org with esmtps (Exim 4.96 #2 (Red Hat Linux)) id 1qyaAc-009pa3-01 for linux-arm-kernel@lists.infradead.org; Thu, 02 Nov 2023 16:03:48 +0000 Received: by mail-pl1-x649.google.com with SMTP id d9443c01a7336-1cc391ca417so9335265ad.0 for ; Thu, 02 Nov 2023 09:03:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1698941024; x=1699545824; darn=lists.infradead.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=2iqXbuRKMyvTfyAi0R6oHncN26PfYTo3tctDI41vJWU=; b=TAFpmrBa6wglHpMXGA7RDVQHBx8jbTjSCpOtOHe/63peXSXd+7bdYSIMpkhHUpjlX6 N9YTqoLFLdX5NnYRptIVJ4/Bb8oqSZfJg5lMZFbf19jUUj8FRwFvw4m5M3Vi1FgUPg+J ZEn4+RHahHbPh0tjTZqfsvXZGF9GnS3QQj0jPHZiDVLYBOC28CmjA/VP6vjUB3iaeotX 526YWgcmVuBTXn7ZblWa4QtHvE0SQu4TON4TGKkchxE90j0U/gzC6cWV7I7gKznhPLxc 2GzSju69r4lbfTP28VK0YVWMSut8VDIjbh33RSBhqkj8Tbn6UIKWxeCB4nKLAu0FxobY 4XaQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1698941024; x=1699545824; 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=2iqXbuRKMyvTfyAi0R6oHncN26PfYTo3tctDI41vJWU=; b=RZU2gcqwVxI4aZe3xqoCfIeImUaarKCJUsGAKqmThNQyfMly8kaCxHml0lEYqnL+Ec FB//WZUoMAJ4lSIO/+gH8603MlD7pwhXNJCM72tf/hoAXzpPUd+H+YB//4S3SCvs6WYL L1E4oJaaGgd0JrZLOQyjJxQ3Ru12EWMtsCoJfBnWv5Vjo1PNw4FqZI3Chwr2NI2NUCB1 pYRBeaBxZo2UZR6HqAHiKjtYglWhSj4WeyVdGWcL5FEMhGvpIa5nMyNWkX/khK6ljX6f C4+0fu2tEPPmo2ZlRnCIYGWhRl/8priQWKrxauNhsiALzFwLpKz3QogriUCpxbIdGE+H KFaQ== X-Gm-Message-State: AOJu0YzwKQ/XQ5OP4A4U3mggW0d+nh24nxCTEtcbeR7PJtzUrRe9uvJY gNr81hNYqoo/5hpyvnezEsWK1o/uvfY= X-Google-Smtp-Source: AGHT+IFM1Nz2eAP9Xmws9JoQjAGGS19Q55KF/ifRxAflcrAzQHFBFIOS8V+26ERzfv1Lm5CG2bErCCPJO0Q= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a17:903:1304:b0:1cc:2ffe:5a33 with SMTP id iy4-20020a170903130400b001cc2ffe5a33mr266680plb.8.1698941024519; Thu, 02 Nov 2023 09:03:44 -0700 (PDT) Date: Thu, 2 Nov 2023 09:03:42 -0700 In-Reply-To: <6642c379-1023-4716-904f-4bbf076744c2@redhat.com> Mime-Version: 1.0 References: <20231027182217.3615211-1-seanjc@google.com> <20231027182217.3615211-17-seanjc@google.com> <6642c379-1023-4716-904f-4bbf076744c2@redhat.com> Message-ID: Subject: Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory From: Sean Christopherson To: Paolo Bonzini Cc: David Matlack , Marc Zyngier , Oliver Upton , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexander Viro , Christian Brauner , "Matthew Wilcox (Oracle)" , Andrew Morton , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Xiaoyao Li , Xu Yilun , Chao Peng , Fuad Tabba , Jarkko Sakkinen , Anish Moorthy , Yu Zhang , Isaku Yamahata , "=?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?=" , Vlastimil Babka , Vishal Annapurve , Ackerley Tng , Maciej Szmigiero , David Hildenbrand , Quentin Perret , Michael Roth , Wang , Liam Merwick , Isaku Yamahata , "Kirill A . Shutemov" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20231102_090346_043766_52B900EB X-CRM114-Status: GOOD ( 25.02 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Nov 02, 2023, Paolo Bonzini wrote: > On 10/31/23 23:39, David Matlack wrote: > > > > Maybe can you sketch out how you see this proposal being extensible to > > > > using guest_memfd for shared mappings? > > > For in-place conversions, e.g. pKVM, no additional guest_memfd is needed. What's > > > missing there is the ability to (safely) mmap() guest_memfd, e.g. KVM needs to > > > ensure there are no outstanding references when converting back to private. > > > > > > For TDX/SNP, assuming we don't find a performant and robust way to do in-place > > > conversions, a second fd+offset pair would be needed. > > Is there a way to support non-in-place conversions within a single guest_memfd? > > For TDX/SNP, you could have a hook from KVM_SET_MEMORY_ATTRIBUTES to guest > memory. The hook would invalidate now-private parts if they have a VMA, > causing a SIGSEGV/EFAULT if the host touches them. > > It would forbid mappings from multiple gfns to a single offset of the > guest_memfd, because then the shared vs. private attribute would be tied to > the offset. This should not be a problem; for example, in the case of SNP, > the RMP already requires a single mapping from host physical address to > guest physical address. I don't see how this can work. It's not a M:1 scenario (where M is multiple gfns), it's a 1:N scenario (wheren N is multiple offsets). The *gfn* doesn't change on a conversion, what needs to change to do non-in-place conversion is the pfn, which is effectively the guest_memfd+offset pair. So yes, we *could* support non-in-place conversions within a single guest_memfd, but it would require a second offset, at which point it makes sense to add a second file descriptor as well. Userspace could still use a single guest_memfd instance, i.e. pass in the same file descriptor but different offsets. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel