Hyprcall code page is specified in the Hyper-V TLFS to be an overlay page, ie., guest chooses a GPA and the host _places_ a page at that location, making it visible to the guest and the existing page becomes inaccessible. Similarly when disabled, the host should _remove_ the overlay and the old page should become visible to the guest. Now, KVM directly patches the instructions into the guest chosen GPA for the hypercall code page. Strictly speaking this is guest memory corruption as the hyper-v TLFS specifies that the underlying page should be preserved. Since the guest seldom moves the hypercall code page around, it didn't see any problems till now. When trying to implement VSM API, we are seeing some exotic use of overlay pages which start expecting the underlying page to be intact. To handle those cases, we need a more generic approach handling these primitives. This patchset tries build an infrastructure for handling overlay pages in general by using the new user space MSR filtering feature of KVM to filter out writes to overlay MSRs, handle them in user space and then forward those writes back to KVM so it gets an opportunity to write contents into the page that was overlaid here. Additionally it does some housekeeping here and there. P.S. This is a follow up to the my initial approach of handling this in kernel, see [1] for discussions. ~ Sid. [1]: https://lore.kernel.org/kvm/20210423090333.21910-1-sidcha@xxxxxxxxx/ v1 -> v2: - Don't allow hypercall page set when guest_os_id is not set. - Address Alex's comments: - Move filtering logic to 6/6 so we don't break bisection. - Reword comment on filters - Provide a default RDMSR handler - Set run->msr.error = 1 to retain existing behaviour Siddharth Chandrasekaran (6): hyper-v: Overlay abstraction for synic event and msg pages hyper-v: Use -1 as invalid overlay address kvm/i386: Stop using cpu->kvm_msr_buf in kvm_put_one_msr() kvm/i386: Avoid multiple calls to check_extension(KVM_CAP_HYPERV) kvm/i386: Add support for user space MSR filtering hyper-v: Handle hypercall code page as an overlay page hw/hyperv/hyperv.c | 116 +++++++++++++++++++-------------- include/hw/hyperv/hyperv.h | 15 +++++ target/i386/kvm/hyperv.c | 101 +++++++++++++++++++++++++++-- target/i386/kvm/hyperv.h | 4 ++ target/i386/kvm/kvm.c | 128 +++++++++++++++++++++++++++++++++++-- target/i386/kvm/kvm_i386.h | 1 + 6 files changed, 305 insertions(+), 60 deletions(-) -- 2.17.1 Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
