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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 13E8AC282DE for ; Thu, 13 Mar 2025 22:38:17 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id C910A280003; Thu, 13 Mar 2025 18:38:15 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C1A12280001; Thu, 13 Mar 2025 18:38:15 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A942D280003; Thu, 13 Mar 2025 18:38:15 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id 871AC280001 for ; Thu, 13 Mar 2025 18:38:15 -0400 (EDT) Received: from smtpin12.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 0EBECA85B7 for ; Thu, 13 Mar 2025 22:38:16 +0000 (UTC) X-FDA: 83217992592.12.6671DA9 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf11.hostedemail.com (Postfix) with ESMTP id A701E40006 for ; Thu, 13 Mar 2025 22:38:13 +0000 (UTC) Authentication-Results: imf11.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="Uo5uW/rS"; spf=pass (imf11.hostedemail.com: domain of peterx@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1741905493; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=aFzr4WLi5EWJd9Q1lyDlYMS58+KhEJhXt4VWqBCulsA=; b=iv7pYRW0c0F251PAyfjBvigU9uDBxKCiHXLCf+Gp9m66jsq6A5fE/dqPeJDxQOr1wzq7OL NFQlsEZdEY+B5zGWvokwzT8NJ5kv82drzBTglETziPH1UREb+DcXeV8LhYQNl98MKFeLFo 9M/UE9vkCOuWAM1yB62fGtHwO4DlztE= ARC-Authentication-Results: i=1; imf11.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="Uo5uW/rS"; spf=pass (imf11.hostedemail.com: domain of peterx@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1741905493; a=rsa-sha256; cv=none; b=yTwSb5MflOVKsPUwquySpN1I3ZYmbmGdwoX1gxWQkn4u88Kk2VyDxuRL2a2ltk5USde7aC nV6zRD04f7DGlBseQrOPgXRDG5XNIZCA3Td2GRbFUUzA3pDmr4ypgcWPsn8c20xdZKvhpg YDhsp+NyEh4/xQRCbXlD5j97wg9fTzw= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1741905493; 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=aFzr4WLi5EWJd9Q1lyDlYMS58+KhEJhXt4VWqBCulsA=; b=Uo5uW/rS1JfaxsQ/BxyMWba6y1yqpluIPhizuTwId+oo2KBAsp8LPuoZohfmzPHQMM2o6y PiFs0hLBeD6y0PIkVXRQ4sh1i4mCDXSSI7rk9AxqXRd6IZJEoyQy+C5m/Qi5mMfnn87hVu 5TjTSBG3kJVRCFU3yhA2q37/aqizN3I= Received: from mail-qt1-f200.google.com (mail-qt1-f200.google.com [209.85.160.200]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-301-esG63EtRMAWqST_wtf4Dpg-1; Thu, 13 Mar 2025 18:38:11 -0400 X-MC-Unique: esG63EtRMAWqST_wtf4Dpg-1 X-Mimecast-MFC-AGG-ID: esG63EtRMAWqST_wtf4Dpg_1741905491 Received: by mail-qt1-f200.google.com with SMTP id d75a77b69052e-476a8aff693so31325721cf.3 for ; Thu, 13 Mar 2025 15:38:11 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1741905491; x=1742510291; 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=aFzr4WLi5EWJd9Q1lyDlYMS58+KhEJhXt4VWqBCulsA=; b=UqDl/IrHOJY2wfHfxrRNFFV5IWGW/gmUL3CjauGuOFDDFdw1+g74D2D88SNfJrtuN+ oVUarTXs8kP/BnedQ8HWntCqhq6/Qo3goWdiJ22stWsBd0BTPq6uR4i1lyPSOi9mR+sG pnreVymjvNpuxs4WqkP+3CqgAbrFz+PVFQIm4uAf3TDmFXqlA5KRMW7LR4yQhqlqiO5a dsoOuMsytc1TpxWpJnbYUfzt5YbpM/ovJRPWeRmvQU9Eg41/E3SQchWx5vBO1EDLsR9g MYYgy/S0Vz5LhEH7QA30is9pO5adTOEAL4gzFMVPSqT9vjw13Bu+6Mlq7jNZd+C55plz /RWg== X-Forwarded-Encrypted: i=1; AJvYcCWPQFAjoIBPTei29d8hE+Mr7iJFT4LIJiOY+bKb77tQk+TBIwRswHzPYk/zXT4vnE6gmVSl74qpnQ==@kvack.org X-Gm-Message-State: AOJu0Yw5+vJik3Oewk6hk68C2ZPT+/h1b/YASeWeBIMB0Bh/NSGw7uXP +urPzkDEINiuMYPACGPlDp13mLOfaEBgFbMV052N1NOiZUuTlS1cD+4+/GHeSUEnDJ4zQhFM2hs Lm2oW3ZwPzs76Gt97Kuou7eLKoZGGs0dB6JsQIy7KbGCNPO0q X-Gm-Gg: ASbGncs7Gge+akAMSJ9xMdCRDCoM0dTqlT8+/FaW5RXZkV+dh3qDLUSku94l9UjlsAQ YO2wD/ZAdLULrlu2a6IIJJ4/5Uf/P+aCKIpo0mBHWF3q0Q/DXCoSO07feF+5A+Z8uyl8oMwvN0B Me2beAK++Fy/bGj7+2An4X04bNEsBuYobYJZl+oXcz8fLzFSTQnn+dPVo1WMnNpvFHp+CY0pP18 NOIAycRfpnOTQOh8zaAUCCdtJbfOG+meHo4/jVaoLURpoOlcs70aQu8XXL6HajdSWDZIGXTx4In lOTGS9A= X-Received: by 2002:ad4:5d65:0:b0:6e8:e8dd:30d1 with SMTP id 6a1803df08f44-6eaeaa5fa9amr2466906d6.22.1741905490924; Thu, 13 Mar 2025 15:38:10 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGBXIseFJxXyT7V3i4JYoBiq0nyGeT0Insspj8UcFpuo+ZEN+6NwuINfQg0ot8cSpabuM7JsA== X-Received: by 2002:ad4:5d65:0:b0:6e8:e8dd:30d1 with SMTP id 6a1803df08f44-6eaeaa5fa9amr2466586d6.22.1741905490665; Thu, 13 Mar 2025 15:38:10 -0700 (PDT) Received: from x1.local ([85.131.185.92]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6eade208b20sm14700356d6.9.2025.03.13.15.38.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 13 Mar 2025 15:38:09 -0700 (PDT) Date: Thu, 13 Mar 2025 18:38:05 -0400 From: Peter Xu To: Nikita Kalyazin Cc: James Houghton , akpm@linux-foundation.org, pbonzini@redhat.com, shuah@kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, david@redhat.com, ryan.roberts@arm.com, quic_eberman@quicinc.com, graf@amazon.de, jgowans@amazon.com, roypat@amazon.co.uk, derekmn@amazon.com, nsaenz@amazon.es, xmarcalx@amazon.com Subject: Re: [RFC PATCH 0/5] KVM: guest_memfd: support for uffd missing Message-ID: References: <9e7536cc-211d-40ca-b458-66d3d8b94b4d@amazon.com> <7c304c72-1f9c-4a5a-910b-02d0f1514b01@amazon.com> <69dc324f-99fb-44ec-8501-086fe7af9d0d@amazon.com> <507e6ad7-2e28-4199-948a-4001e0d6f421@amazon.com> MIME-Version: 1.0 In-Reply-To: <507e6ad7-2e28-4199-948a-4001e0d6f421@amazon.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: UaVJ1qEpT-UXomxL86bzwFw2jCwj80ehcRK4BDLw2Fo_1741905491 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: A701E40006 X-Stat-Signature: ng9basmoczd6f7xt4fgce7wfh6p64oaj X-HE-Tag: 1741905493-862776 X-HE-Meta: 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 GV9HxG23 5rke9GG6gK9a6kqn80zjeRLm4VTTLvh4cwft4Y8xoTITUPUzpkKWC0cWOElhP6jcNfUVEG7xlARJKEnu5ooNHISPIVblnWP4Tji7y+93t2niNOA0ahsFkIeqyWmfxobdIPjoEetaRfHwBr0EyFJoUEhRs8y+LK0rrgonR1tDC2CS3b7LXXurx3vbs+kWbUAx9WfhUac2RFu3txDBlK/bwOE41v1u8N7AaWvQS4ibrr05gfO/0kb3JhmEL4DnnUyGDIpTv3422XIqsb1z4rLpTHNjhc1gwwvenp19pSF+hSK3F/u+MJuBn+1jnE50LrGKD5QdcduSGzZmRGHJyHEqX/YZuFEZd1JZsyqGk5v95HoBqfxI0E93XGImXrwhELVjNNfcoELBGiP73Nu/3QsL/uR5e/16Z9nWi68J+Tn7/jCuS7yAHcqUC/7cqVnUe5l2EyepVCEPeExRPLW4M/cBd7NUahd3RpchAsTkKMSkAKdsGcJtthPXRt0TXE0BxEGDNlA0TKJ97pjW+ROZ3wSQBW5r+/eivA59h0CqkZgE4ihii8bk= X-Bogosity: Ham, tests=bogofilter, spamicity=0.002964, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Mar 13, 2025 at 10:13:23PM +0000, Nikita Kalyazin wrote: > Yes, that's right, mmap() + memcpy() is functionally sufficient. write() is > an optimisation. Most of the pages in guest_memfd are only ever accessed by > the vCPU (not userspace) via TDP (stage-2 pagetables) so they don't need > userspace pagetables set up. By using write() we can avoid VMA faults, > installing corresponding PTEs and double page initialisation we discussed > earlier. The optimised path only contains pagecache population via write(). > Even TDP faults can be avoided if using KVM prefaulting API [1]. > > [1] https://docs.kernel.org/virt/kvm/api.html#kvm-pre-fault-memory Could you elaborate why VMA faults matters in perf? If we're talking about postcopy-like migrations on top of KVM guest-memfd, IIUC the VMAs can be pre-faulted too just like the TDP pgtables, e.g. with MADV_POPULATE_WRITE. Normally, AFAIU userapp optimizes IOs the other way round.. to change write()s into mmap()s, which at least avoids one round of copy. For postcopy using minor traps (and since guest-memfd is always shared and non-private..), it's also possible to feed the mmap()ed VAs to NIC as buffers (e.g. in recvmsg(), for example, as part of iovec[]), and as long as the mmap()ed ranges are not registered by KVM memslots, there's no concern on non-atomic copy. Thanks, -- Peter Xu