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
