KVM: selftests: Add x86-only selftest for private memory conversions

Add a selftest to exercise implicit/explicit conversion functionality
within KVM and verify:

 - Shared memory is visible to host userspace
 - Private memory is not visible to host userspace
 - Host userspace and guest can communicate over shared memory
 - Data in shared backing is preserved across conversions (test's
   host userspace doesn't free the data)
 - Private memory is bound to the lifetime of the VM

Ideally, KVM's selftests infrastructure would be reworked to allow backing
a single region of guest memory with multiple memslots for _all_ backing
types and shapes, i.e. ideally the code for using a single backing fd
across multiple memslots would work for "regular" memory as well.  But
sadly, support for KVM_CREATE_GUEST_MEMFD has languished for far too long,
and overhauling selftests' memslots infrastructure would likely open a can
of worms, i.e. delay things even further.

In addition to the more obvious tests, verify that PUNCH_HOLE actually
frees memory.  Directly verifying that KVM frees memory is impractical, if
it's even possible, so instead indirectly verify memory is freed by
asserting that the guest reads zeroes after a PUNCH_HOLE.  E.g. if KVM
zaps SPTEs but doesn't actually punch a hole in the inode, the subsequent
read will still see the previous value.  And obviously punching a hole
shouldn't cause explosions.

Let the user specify the number of memslots in the private mem conversion
test, i.e. don't require the number of memslots to be '1' or "nr_vcpus".
Creating more memslots than vCPUs is particularly interesting, e.g. it can
result in a single KVM_SET_MEMORY_ATTRIBUTES spanning multiple memslots.
To keep the math reasonable, align each vCPU's chunk to at least 2MiB (the
size is 2MiB+4KiB), and require the total size to be cleanly divisible by
the number of memslots.  The goal is to be able to validate that KVM plays
nice with multiple memslots, being able to create a truly arbitrary number
of memslots doesn't add meaningful value, i.e. isn't worth the cost.

Intentionally don't take a requirement on KVM_CAP_GUEST_MEMFD,
KVM_CAP_MEMORY_FAULT_INFO, KVM_MEMORY_ATTRIBUTE_PRIVATE, etc., as it's a
KVM bug to advertise KVM_X86_SW_PROTECTED_VM without its prerequisites.

Signed-off-by: Vishal Annapurve <vannapurve@google.com>
Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20231027182217.3615211-32-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>