On Mon, Oct 17, 2022 at 06:39:06PM +0200, Gupta, Pankaj wrote: > On 10/17/2022 6:19 PM, Kirill A . Shutemov wrote: > > On Mon, Oct 17, 2022 at 03:00:21PM +0200, Vlastimil Babka wrote: > > > On 9/15/22 16:29, Chao Peng wrote: > > > > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> > > > > > > > > KVM can use memfd-provided memory for guest memory. For normal userspace > > > > accessible memory, KVM userspace (e.g. QEMU) mmaps the memfd into its > > > > virtual address space and then tells KVM to use the virtual address to > > > > setup the mapping in the secondary page table (e.g. EPT). > > > > > > > > With confidential computing technologies like Intel TDX, the > > > > memfd-provided memory may be encrypted with special key for special > > > > software domain (e.g. KVM guest) and is not expected to be directly > > > > accessed by userspace. Precisely, userspace access to such encrypted > > > > memory may lead to host crash so it should be prevented. > > > > > > > > This patch introduces userspace inaccessible memfd (created with > > > > MFD_INACCESSIBLE). Its memory is inaccessible from userspace through > > > > ordinary MMU access (e.g. read/write/mmap) but can be accessed via > > > > in-kernel interface so KVM can directly interact with core-mm without > > > > the need to map the memory into KVM userspace. > > > > > > > > It provides semantics required for KVM guest private(encrypted) memory > > > > support that a file descriptor with this flag set is going to be used as > > > > the source of guest memory in confidential computing environments such > > > > as Intel TDX/AMD SEV. > > > > > > > > KVM userspace is still in charge of the lifecycle of the memfd. It > > > > should pass the opened fd to KVM. KVM uses the kernel APIs newly added > > > > in this patch to obtain the physical memory address and then populate > > > > the secondary page table entries. > > > > > > > > The userspace inaccessible memfd can be fallocate-ed and hole-punched > > > > from userspace. When hole-punching happens, KVM can get notified through > > > > inaccessible_notifier it then gets chance to remove any mapped entries > > > > of the range in the secondary page tables. > > > > > > > > The userspace inaccessible memfd itself is implemented as a shim layer > > > > on top of real memory file systems like tmpfs/hugetlbfs but this patch > > > > only implemented tmpfs. The allocated memory is currently marked as > > > > unmovable and unevictable, this is required for current confidential > > > > usage. But in future this might be changed. > > > > > > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > > > > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> > > > > --- > > > > > > ... > > > > > > > +static long inaccessible_fallocate(struct file *file, int mode, > > > > + loff_t offset, loff_t len) > > > > +{ > > > > + struct inaccessible_data *data = file->f_mapping->private_data; > > > > + struct file *memfd = data->memfd; > > > > + int ret; > > > > + > > > > + if (mode & FALLOC_FL_PUNCH_HOLE) { > > > > + if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len)) > > > > + return -EINVAL; > > > > + } > > > > + > > > > + ret = memfd->f_op->fallocate(memfd, mode, offset, len); > > > > + inaccessible_notifier_invalidate(data, offset, offset + len); > > > > > > Wonder if invalidate should precede the actual hole punch, otherwise we open > > > a window where the page tables point to memory no longer valid? > > > > Yes, you are right. Thanks for catching this. > > I also noticed this. But then thought the memory would be anyways zeroed > (hole punched) before this call? Hole punching can free pages, given that offset/len covers full page. -- Kiryl Shutsemau / Kirill A. Shutemov