On Mon, Jun 08, 2026 at 04:55:19PM +0200, David Hildenbrand (Arm) wrote: > On 6/8/26 16:44, Matthew Wilcox wrote: > > On Mon, Jun 08, 2026 at 04:37:03PM +0200, David Hildenbrand (Arm) wrote: > >> On 6/8/26 16:31, Matthew Wilcox wrote: > >>> > >>> What I don't understand is how the kernel page allocator needs to know > >>> the user address in order to effectively zero it, but the hypervisor is > >>> able to zero the page without knowing the user address. It feels like > >>> somebody has x86-centric thinking where cache colouring doesn't matter. > >> > >> (not commenting on the icache dache mess we have to drag along) > > > > Well, that was kind of the point of this email ... I did ask the > > question you're answering in a different email so let me respond > > to that too. > > Now I'm confused :) > > > > >> The thing is that with free-page-reporting the memory is already zeroed by > >> the > >> hypervisor as part of discarding that memory previously (e.g., > >> MADV_DONTNEED) > >> and allocating fresh pages on re-access. > >> > >> So it's not a question of "why is the hypervisor zeroing less > >> efficiently", as > >> zeroing is just a side-product of reclaiming that memory in the first > >> place. > > > > We definitely have users who don't want the guest to trust the > > hypervisor. So how do they disable this optimisation? > > Right, I don't think we currently have a toggle to disable free page > reporting. > So IIUC, this optimization would similarly automatically get enabled if the > hypervisor advertises it. > > -- > Cheers, > > David
Not as the patchset stands: [PATCH v10 35/37] virtio_balloon: disable reporting zeroed optimization for confidential guests disables it. -- MST
