From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 88FBA3164C5; Wed, 15 Jul 2026 09:49:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784108971; cv=none; b=Q+S5gvb7LOJVsLlqfKr2vtq9s3ydsS1oIdQZnkXyFjVUhz7RZ2ByYHaIug9azwFaT9c3oZnbL+FzNZOpqdqTm06YkaDYhLUwc47ciwVTenubWa4hGNWix5SANJchomI4oDMFsUrjodE7aAUseq2ycaTDhx/tadK1jDgbaNXuhMs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784108971; c=relaxed/simple; bh=L9ZM9WGLctd1y/Irc9n+QeoIlbBDdN94DCSH+2nYh4o=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=c4FeDHTrKOVK3HBCqu/LdcehqL+1OE0TmHgmdUrNmez5B8TM248Co9MpYqRZU3oH0jI+wEf6rJLS2/rIr1CeJV+Ft8ISZEGy+dQDJPCWiiTrqr5xdWOpl0XvPFadSfsACMXNep/PnIy47j458JEhIyBtmNRxm16kDD5PSJwWYdU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fe8kwNlI; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="fe8kwNlI" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E183C1F000E9; Wed, 15 Jul 2026 09:49:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784108970; bh=MNXk5Xsq0/VzXk4rQ9wgNzCQfwCZmMN1C9U5Bm+mdc8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fe8kwNlISuqGkpWHze1UvqZvzqnDB0PrJ+TR+DVKi7YJWjOiqXbusRmpsQ486QMI5 KKtKH/UZqtkBPIqT1NTCqxoG7TfdRRVldQrlXNNG9/Xz4UDziadlzRPiw12byYDxrc SCPjU2vnuHAYEJ5pbqQDS0QUZimyRWi02ajjYkcuj2m2zJS2AtvEQXKDKNn1UCZUmH HsnDa9ew2wMlP7sGr3JF106xnogYHjPAn8zp0PmuwaBDFFPaLxolwXoAo1zZTmpsk8 IWsIz49cWiX3CywlsLLxXiinfxt5gtXk7QPL1kd38JbCx2e75lSEkNT6An61ooMhi/ y8vONKkrt5BnQ== Message-ID: Date: Wed, 15 Jul 2026 11:49:26 +0200 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: [PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration To: Pankaj Gupta , seanjc@google.com, pbonzini@redhat.com, tglx@kernel.org, bp@alien8.de, mingo@redhat.com, dave.hansen@linux.intel.com Cc: x86@kernel.org, thomas.lendacky@amd.com, hpa@zytor.com, yangge1116@126.com, ljs@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260715063626.65899-1-pankaj.gupta@amd.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/15/26 08:36, Pankaj Gupta wrote: > Commit 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering > encrypted memory regions") added FOLL_LONGTERM to > sev_mem_enc_register_region() so anonymous guest RAM is migrated out of > MIGRATE_CMA/ZONE_MOVABLE before a long term pin. It also kept > FOLL_WRITE on the pin. > > Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed > guest memory, such as virtio-pmem host memory-backend-file mappings > (MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file > mappings since: > > commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings") > commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings"). > > Region registration only requires long-term pin to prevent page migration and > does not write through this GUP pin. > > Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM. Worth mentioning here something like "In the past, FOLL_WRITE was required to trigger CoW unsharing, making sure that we don't end up replacing the page in the page tables during a later write fault after already having pinned a (shared) page in MAP_PRIVATE mappings. FOLL_LONGTERM does that nowadays (see gup_must_unshare()) even without FOLL_WRITE. Given that SEV only pins RAM for XYZ and doesn't actually write to the pinned pages, we can just drop the FOLL_WRITE" Fill out XYZ :) In general, LGTM Acked-by: David Hildenbrand (Arm) -- Cheers, David