From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f202.google.com (mail-pl1-f202.google.com [209.85.214.202]) (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 466A91D69C for ; Thu, 2 Nov 2023 16:03:44 +0000 (UTC) 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="ePyDCfOS" Received: by mail-pl1-f202.google.com with SMTP id d9443c01a7336-1cc1682607eso9219505ad.1 for ; Thu, 02 Nov 2023 09:03:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1698941024; x=1699545824; darn=lists.linux.dev; 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=ePyDCfOS9ww1QuylL8HY8IHPu/x1Bw906v2cMTJ0kCsunQvLorrehIKJEMAlehRPJ5 wqhQxZ+1Sv7hgfFp3kggG1xFsA2HRW8P6Y/eDGOPGr3+KG29DB14O+ua+aMI+Y2KkjQw t0RShHgpCJXmqVOHjHyaF5BWBf+x/VzopW1yB5jHiLA4BYYwWu0vW/ob29dSyi0iZRZG Da3H/+3z1tXO0zNB0G3Nw/De4IXE8Jq/5C+WZC56IgQ8ubJd/DlFyj4VI0ZucYzg6mE6 fjWWaqEshIbDCr9ZN6kAgc3O7E4Q9P8ueuaLeMZQUh2yRbXwaKhDK6XA9Kr+kdD3tzPn 0naw== 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=bpfOy0+c1LoJD3lJeMnGXP1wVZhQYBQUZ+sCIi92oUVap1eANL3JgMrZLqC82AuTu4 xNlsx96T9bvNZGBx5K/AE/Fjh+lZsx9+1cna44KcGJnDIqVys2SYVgj0GsORT/NMwj32 M5HJj5FOOzzHzzEvbDh/vakmYdYIIOrdfcSbDAHYcz2Ye5E0Cyebm9rJ+ByLqoVcIMQ5 MwTAeS0PgvzSL3nlVUcb8lb0DXtSLAA5an+RdfQt+g1eeRILp3ZA5PDWlN33LZy3Qrvx mEziCUfxsj6pSNjHr37vBI5c3jh1SNmC9mi+vtWcKuG+vdtksKuY9wnMJyvxqNnZcmKj lmuA== X-Gm-Message-State: AOJu0YygxSwxPmcak37IfwvVgU9KiLDLTHAr8Ih2YNZVfv6RMKeXR9iJ W7p3mIj9vurTXWQckD5lMizZ7Ufl7hY= 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> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: 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" Content-Type: text/plain; charset="us-ascii" 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.