Re: [PATCH] KVM: hyper-v: Add new exit reason HYPERV_OVERLAY

[Siddharth Chandrasekaran]

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.



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