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Tue, 27 Oct 2026 03:29:34 -0700 (PDT) Date: Tue, 27 Oct 2026 06:29:29 -0400 From: "Michael S. Tsirkin" To: Kevin Loughlin Cc: David Hildenbrand , Jason Wang , Eugenio =?iso-8859-1?Q?P=E9rez?= , Xuan Zhuo , Ard Biesheuvel , Kiryl Shutsemau , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Tom Lendacky , Ashish Kalra , Michael Roth , Rick Edgecombe , Andrew Morton , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Alexander Duyck , virtualization@lists.linux.dev, linux-coco@lists.linux.dev, linux-efi@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sidharth Telang , Punit Salian , Peter Gonda , Ackerley Tng , Jacky Li , Steve Rutherford , David Skidmore Subject: Re: [PATCH 00/13] virtio_balloon, efi/unaccepted: support memory ballooning and free page reporting in CoCo guests Message-ID: <20261027051245-mutt-send-email-mst@kernel.org> References: <20261027025616.1632690-1-kevinloughlin@google.com> X-Mailing-List: linux-coco@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 In-Reply-To: <20261027025616.1632690-1-kevinloughlin@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: -_56ThqYv5aAbLg12TvCuAq87RC7BT2AylHa5kMazlY_1793096976 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, Oct 27, 2026 at 02:56:03AM +0000, Kevin Loughlin wrote: > This series enables virtio-balloon free page reporting and memory balloon > inflation/deflation in Confidential Computing (CoCo) guests, starting > with architectural hooks implemented for AMD SEV variants, in order to > support dynamic CoCo guest resizing. > > Today, memory ballooning and free page reporting are essentially unusable > in CoCo guests due to three main roadblocks: For me, the bigger question is not whether we can, but whether we should. Considerations I would like to see addressed: Security, e.g.: does knowing which memory is free leak information from the guest? If no, why not? Consider CONFIG_SHUFFLE_PAGE_ALLOCATOR, RANDSTRUCT, etc. correctness, e.g.: Linux has init on free (and default on windows), if initialized pages are dropped and host can not be trusted, they have to be re-initialized. Is this handled? I don't see it in code. Also, performance, e.g.: How much does this re-initialization cost? Management: it was simple with balloon off unconditionally. Now we are in murkier waters with various features offering various performance/security tradeoffs. What if some guests want the tradeoffs but others don't? Documentation: for all of the above. -- MST