On Fri, Apr 23, 2021 at 11:24:04AM +0200, Alexander Graf wrote: > > > On 23.04.21 11:03, Siddharth Chandrasekaran wrote: > > Hypercall 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. > > > > Currently KVM directly patches the hypercall code into the guest chosen > > GPA. Since the guest seldom moves the hypercall code page around, it > > doesn't see any problems even though we are corrupting the exiting data > > in that GPA. > > > > VSM API introduces more complex overlay workflows during VTL switches > > where the guest starts to expect that the existing page is intact. This > > means we need a more generic approach to handling overlay pages: add a > > new exit reason KVM_EXIT_HYPERV_OVERLAY that exits to userspace with the > > expectation that a page gets overlaid there. > > I can see how that may get interesting for other overlay pages later, but > this one in particular is just an MSR write, no? Is there any reason we > can't just use the user space MSR handling logic instead? > > What's missing then is a way to pull the hcall page contents from KVM. But > even there I'm not convinced that KVM should be the reference point for its > contents. Isn't user space in an as good position to assemble it? Makes sense. Let me explore that route and get back to you. > > > > In the interest of maintaing userspace exposed behaviour, add a new KVM > > capability to allow the VMMs to enable this if they can handle the > > hypercall page in userspace. > > > > Signed-off-by: Siddharth Chandrasekaran <sidcha@xxxxxxxxx> > > > > CR: https://code.amazon.com/reviews/CR-49011379 > > Please remove this line from upstream submissions :). I noticed it a bit late (a tooling gap). You shouldn't see this in any of my future patches. > > --- > > arch/x86/include/asm/kvm_host.h | 4 ++++ > > arch/x86/kvm/hyperv.c | 25 ++++++++++++++++++++++--- > > arch/x86/kvm/x86.c | 5 +++++ > > include/uapi/linux/kvm.h | 10 ++++++++++ > > You're modifying / adding a user space API. Please make sure to update the > documentation in Documentation/virt/kvm/api.rst when you do that. Ack. Will add it. 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
