On Fri, Feb 09, 2024 at 07:13:13AM -0800, Sean Christopherson wrote: > On Fri, Feb 09, 2024, Steven Price wrote: > > >> One option that I've considered is to implement a seperate CCA ioctl to > > >> notify KVM whether the memory should be mapped protected. > > > > > > That's what KVM_SET_MEMORY_ATTRIBUTES+KVM_MEMORY_ATTRIBUTE_PRIVATE is for, no? > > > > Sorry, I really didn't explain that well. Yes effectively this is the > > attribute flag, but there's corner cases for destruction of the VM. My > > thought was that if the VMM wanted to tear down part of the protected > > range (without making it shared) then a separate ioctl would be needed > > to notify KVM of the unmap. > > No new uAPI should be needed, because the only scenario time a benign VMM should > do this is if the guest also knows the memory is being removed, in which case > PUNCH_HOLE will suffice. > > > >> This 'solves' the problem nicely except for the case where the VMM > > >> deliberately punches holes in memory which the guest is using. > > > > > > I don't see what problem there is to solve in this case. PUNCH_HOLE is destructive, > > > so don't do that. > > > > A well behaving VMM wouldn't PUNCH_HOLE when the guest is using it, but > > my concern here is a VMM which is trying to break the host. In this case > > either the PUNCH_HOLE needs to fail, or we actually need to recover the > > memory from the guest (effectively killing the guest in the process). > > The latter. IIRC, we talked about this exact case somewhere in the hour-long > rambling discussion on guest_memfd at PUCK[1]. And we've definitely discussed > this multiple times on-list, though I don't know that there is a single thread > that captures the entire plan. > > The TL;DR is that gmem will invoke an arch hook for every "struct kvm_gmem" > instance that's attached to a given guest_memfd inode when a page is being fully > removed, i.e. when a page is being freed back to the normal memory pool. Something > like this proposed SNP patch[2]. > > Mike, do have WIP patches you can share? Sorry, I missed this query earlier. I'm a bit confused though, I thought the kvm_arch_gmem_invalidate() hook provided in this patch was what we ended up agreeing on during the PUCK call in question. There was an open question about what to do if a use-case came along where we needed to pass additional parameters to kvm_arch_gmem_invalidate() other than just the start/end PFN range for the pages being freed, but we'd determined that SNP and TDX did not currently need this, so I didn't have any changes planned in this regard. If we now have such a need, what we had proposed was to modify __filemap_remove_folio()/page_cache_delete() to defer setting folio->mapping to NULL so that we could still access it in kvm_gmem_free_folio() so that we can still access mapping->i_private_list to get the list of gmem/KVM instances and pass them on via kvm_arch_gmem_invalidate(). So that's doable, but it's not clear from this discussion that that's needed. If the idea to block/kill the guest if VMM tries to hole-punch, and ARM CCA already has plans to wire up the shared/private flags in kvm_unmap_gfn_range(), wouldn't that have all the information needed to kill that guest? At that point, kvm_gmem_free_folio() can handle additional per-page cleanup (with additional gmem/KVM info plumbed in if necessary). -Mike [1] https://lore.kernel.org/kvm/20240202230611.351544-1-seanjc@xxxxxxxxxx/T/ > > [1] https://drive.google.com/corp/drive/folders/116YTH1h9yBZmjqeJc03cV4_AhSe-VBkc?resourcekey=0-sOGeFEUi60-znJJmZBsTHQ > [2] https://lore.kernel.org/all/20231230172351.574091-30-michael.roth@xxxxxxx
