From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp-fw-2101.amazon.com (smtp-fw-2101.amazon.com [72.21.196.25]) (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 320D64503A; Thu, 1 Aug 2024 10:30:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=72.21.196.25 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722508231; cv=none; b=a0Tnzgdw5TMb8qkG/emuPZHQatDga5VxpRqFHNPsGO3pGAeDQIiYEdbq38bknH5Pbe/F5jruLOfaanhwe5sbq9b+I9s9vr1iK3nCmiKfwWG6CcXIiBT4SU4m4B0OiouuWh+B13LKvnglYvMQt+X2hAJ1f/q9NhDhFFKcOWXu5M0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722508231; c=relaxed/simple; bh=TeXgvZtmVSsWOyWvwGwQNHpvFHcawwpfpA+OnYmmyEg=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=ZVKqfUonXyIT4AfOjqVTa6+lb9yI7bgGu9cUviihJGaTXsTR8yJgY8DoWvA1uxtGZUppVTBQaLGp7GM8FqmzMswccBvBJjSj9KazxakiCb14Yo3/O9X5+ztMLz4tiixTvi/GTLPgLzzXG4vXBF0HkKiVXqyUiIZ/duimB8rf6JY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.co.uk; spf=pass smtp.mailfrom=amazon.co.uk; dkim=pass (1024-bit key) header.d=amazon.co.uk header.i=@amazon.co.uk header.b=uE9BADz4; arc=none smtp.client-ip=72.21.196.25 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.co.uk Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=amazon.co.uk Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=amazon.co.uk header.i=@amazon.co.uk header.b="uE9BADz4" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.co.uk; i=@amazon.co.uk; q=dns/txt; s=amazon201209; t=1722508229; x=1754044229; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=OVVbgg3i7ObhzDXrRl+WHtJky+rbfnkhgnMTtTNFVvM=; b=uE9BADz43zxPo0mxslQCLpPSug259uDOe225dLLHnh8VPUOJHT09NVIF b55yT2g3Ss5cFmFNFBt34T3k5G/HGPPyrgjyexsCaA2PdyrjCTqPXBNvi 4B1nprGZN62Iar9EsVF6k4ZN5u84KDjl0geThyPoMMyUXy6PYyE52v1Gd g=; X-IronPort-AV: E=Sophos;i="6.09,254,1716249600"; d="scan'208";a="418659193" Received: from iad12-co-svc-p1-lb1-vlan3.amazon.com (HELO smtpout.prod.us-east-1.prod.farcaster.email.amazon.dev) ([10.43.8.6]) by smtp-border-fw-2101.iad2.amazon.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Aug 2024 10:30:26 +0000 Received: from EX19MTAUEA001.ant.amazon.com [10.0.0.204:41573] by smtpin.naws.us-east-1.prod.farcaster.email.amazon.dev [10.0.0.142:2525] with esmtp (Farcaster) id af2fe4b0-83dd-4544-a4ed-27a0ca73effc; Thu, 1 Aug 2024 10:30:25 +0000 (UTC) X-Farcaster-Flow-ID: af2fe4b0-83dd-4544-a4ed-27a0ca73effc Received: from EX19D008UEA004.ant.amazon.com (10.252.134.191) by EX19MTAUEA001.ant.amazon.com (10.252.134.203) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.1258.34; Thu, 1 Aug 2024 10:30:25 +0000 Received: from EX19MTAUEB001.ant.amazon.com (10.252.135.35) by EX19D008UEA004.ant.amazon.com (10.252.134.191) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.1258.34; Thu, 1 Aug 2024 10:30:25 +0000 Received: from [127.0.0.1] (172.19.88.180) by mail-relay.amazon.com (10.252.135.35) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.1258.34 via Frontend Transport; Thu, 1 Aug 2024 10:30:22 +0000 Message-ID: Date: Thu, 1 Aug 2024 11:30:21 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 8/8] kvm: gmem: Allow restricted userspace mappings To: David Hildenbrand , , Fuad Tabba CC: , , , , , , , , , , , , , , , , , , , , James Gowans References: <20240709132041.3625501-1-roypat@amazon.co.uk> <20240709132041.3625501-9-roypat@amazon.co.uk> <47ce1b10-e031-4ac1-b88f-9d4194533745@redhat.com> From: Patrick Roy Content-Language: en-US Autocrypt: addr=roypat@amazon.co.uk; keydata= xjMEY0UgYhYJKwYBBAHaRw8BAQdA7lj+ADr5b96qBcdINFVJSOg8RGtKthL5x77F2ABMh4PN NVBhdHJpY2sgUm95IChHaXRodWIga2V5IGFtYXpvbikgPHJveXBhdEBhbWF6b24uY28udWs+ wpMEExYKADsWIQQ5DAcjaM+IvmZPLohVg4tqeAbEAgUCY0UgYgIbAwULCQgHAgIiAgYVCgkI CwIEFgIDAQIeBwIXgAAKCRBVg4tqeAbEAmQKAQC1jMl/KT9pQHEdALF7SA1iJ9tpA5ppl1J9 AOIP7Nr9SwD/fvIWkq0QDnq69eK7HqW14CA7AToCF6NBqZ8r7ksi+QLOOARjRSBiEgorBgEE AZdVAQUBAQdAqoMhGmiXJ3DMGeXrlaDA+v/aF/ah7ARbFV4ukHyz+CkDAQgHwngEGBYKACAW IQQ5DAcjaM+IvmZPLohVg4tqeAbEAgUCY0UgYgIbDAAKCRBVg4tqeAbEAtjHAQDkh5jZRIsZ 7JMNkPMSCd5PuSy0/Gdx8LGgsxxPMZwePgEAn5Tnh4fVbf00esnoK588bYQgJBioXtuXhtom 8hlxFQM= In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit On Tue, 2024-07-30 at 11:15 +0100, David Hildenbrand wrote: >>> Hi, >>> >>> sorry for the late reply. Yes, you could have joined .... too late. >> >> No worries, I did end up joining to listen in to y'all's discussion >> anyway :) > > Sorry for the late reply :( No worries :) >> >>> There will be a summary posted soon. So far the agreement is that we're >>> planning on allowing shared memory as part guest_memfd, and will allow >>> that to get mapped and pinned. Private memory is not going to get mapped >>> and pinned. >>> >>> If we have to disallow pinning of shared memory on top for some use >>> cases (i.e., no directmap), I assume that could be added. >>> >>>> >>>>> Note that just from staring at this commit, I don't understand the >>>>> motivation *why* we would want to do that. >>>> >>>> Fair - I admittedly didn't get into that as much as I probably should >>>> have. In our usecase, we do not have anything that pKVM would (I think) >>>> call "guest-private" memory. I think our memory can be better described >>>> as guest-owned, but always shared with the VMM (e.g. userspace), but >>>> ideally never shared with the host kernel. This model lets us do a lot >>>> of simplifying assumptions: Things like I/O can be handled in userspace >>>> without the guest explicitly sharing I/O buffers (which is not exactly >>>> what we would want long-term anyway, as sharing in the guest_memfd >>>> context means sharing with the host kernel), we can easily do VM >>>> snapshotting without needing things like TDX's TDH.EXPORT.MEM APIs, etc. >>> >>> Okay, so essentially you would want to use guest_memfd to only contain >>> shard memory and disallow any pinning like for secretmem. >> >> Yeah, this is pretty much what I thought we wanted before listening in >> on Wednesday. >> >> I've actually be thinking about this some more since then though. With >> hugepages, if the VM is backed by, say, 2M pages, our on-demand direct >> map insertion approach runs into the same problem that CoCo VMs have >> when they're backed by hugepages: How to deal with the guest only >> sharing a 4K range in a hugepage? If we want to restore the direct map >> for e.g. the page containing kvm-clock data, then we can't simply go >> ahead and restore the direct map for the entire 2M page, because there >> very well might be stuff in the other 511 small guest pages that we >> really do not want in the direct map. And we can't even take the > > Right, you'd only want to restore the direct map for a fragment. Or > dynamically map that fragment using kmap where required (as raised by > Vlastimil). Can the kmap approach work if the memory is supposed to be GUP-able? >> approach of letting the guest deal with the problem, because here >> "sharing" is driven by the host, not the guest, so the guest cannot >> possibly know that it maybe should avoid putting stuff it doesn't want >> shared into those remaining 511 pages! To me that sounds a lot like the >> whole "breaking down huge folios to allow GUP to only some parts of it" >> thing mentioned on Wednesday. > > Yes. While it would be one logical huge page, it would be exposed to the > remainder of the kernel as 512 individual pages. > >> >> Now, if we instead treat "guest memory without direct map entries" as >> "private", and "guest memory with direct map entries" as "shared", then >> the above will be solved by whatever mechanism allows gupping/mapping of >> only the "shared" parts of huge folios, IIUC. The fact that GUP is then >> also allowed for the "shared" parts is not actually a problem for us - >> we went down the route of disabling GUP altogether here because based on >> [1] it sounded like GUP for anything gmem related would never happen. > > Right. Might there also be a case for removing the directmap for shared > memory or is that not really a requirement so far? No, not really - we would only mark as "shared" memory that _needs_ to be in the direct map for functional reasons (e.g. MMIO instruction emulation, etc.). >> But after something is re-inserted into the direct map, we don't very >> much care if it can be GUP-ed or not. In fact, allowing GUP for the >> shared parts probably makes some things easier for us, as we can then do >> I/O without bounce buffers by just in-place converting I/O-buffers to >> shared, and then treating that shared slice of guest_memfd the same way >> we treat traditional guest memory today. > > Yes. > >> In a very far-off future, we'd >> like to be able to do I/O without ever reinserting pages into the direct >> map, but I don't think adopting this private/shared model for gmem would >> block us from doing that? > > How would that I/O get triggered? GUP would require the directmap. I was hoping that this "phyr" thing Matthew has been talking about [1] would allow somehow doing I/O without direct map entries/GUP, but maybe I am misunderstanding something. >> >> Although all of this does hinge on us being able to do the in-place >> shared/private conversion without any guest involvement. Do you envision >> that to be possible? > > Who would trigger the conversion and how? I don't see a reason why -- > for your use case -- user space shouldn't be able to trigger conversion > private <-> shared. At least nothing fundamental comes to mind that > would prohibit that. Either KVM itself would trigger the conversions whenever it wants to access gmem (e.g. each place in this series where there is a set_direct_map_{invalid,default} it would do a shared/private conversion), or userspace would do it via some syscall/ioctl (the one place I can think of right now is I/O, where the VMM receives a virtio buffer from the guest and converts it from private to shared in-place. Although I guess 2 syscalls for each I/O operation aren't great perf-wise, so maybe swiotlb still wins out here?). I actually see that Fuad just posted an RFC series that implements the basic shared/private handling [2], so will probably also comment about this over there after I had a closer look :) > -- > Cheers, > > David / dhildenb Best, Patrick [1]: https://lore.kernel.org/netdev/Yd0IeK5s%2FE0fuWqn@casper.infradead.org/T/ [2]: https://lore.kernel.org/kvm/20240801090117.3841080-1-tabba@google.com/T/#t