> On Oct 19, 2020, at 11:19 PM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > For removing the userspace mapping, use a trick similar to what NUMA > balancing does: convert memory that belongs to KVM memory slots to > PROT_NONE: all existing entries converted to PROT_NONE with mprotect() and > the newly faulted in pages get PROT_NONE from the updated vm_page_prot. > The new VMA flag -- VM_KVM_PROTECTED -- indicates that the pages in the > VMA must be treated in a special way in the GUP and fault paths. The flag > allows GUP to return the page even though it is mapped with PROT_NONE, but > only if the new GUP flag -- FOLL_KVM -- is specified. Any userspace access > to the memory would result in SIGBUS. Any GUP access without FOLL_KVM > would result in -EFAULT. > I definitely like the direction this patchset is going in, and I think that allowing KVM guests to have memory that is inaccessible to QEMU is a great idea. I do wonder, though: do we really want to do this with these PROT_NONE tricks, or should we actually come up with a way to have KVM guest map memory that isn't mapped into QEMU's mm_struct at all? As an example of the latter, I mean something a bit like this: https://lkml.kernel.org/r/CALCETrUSUp_7svg8EHNTk3nQ0x9sdzMCU=h8G-Sy6=SODq5GHg@xxxxxxxxxxxxxx I don't mean to say that this is a requirement of any kind of protected memory like this, but I do think we should understand the tradeoffs, in terms of what a full implementation looks like, the effort and time frames involved, and the maintenance burden of supporting whatever gets merged going forward.
