[PATCH v2 0/2] x86/hyperv: Bug fix and enhancement

[Sean Christopherson <seanjc@google.com>]

Patch 01 is a fix for a NULL pointer deref that I ran into with a bad VMM
configuration.  That specific error path is remedied by patch 02, but
Hyper-V can still end up in an inactive state if a memory allocation fails.

Patch 02 effectively makes the required MSRs mandatory for recognizing
Hyper-V at all.

Some versions of QEMU prior to ~6.0 make it all too easy to advertise
Hyper-V and a slew of features without advertising the Hyper-V HYPERCALL
MSR, e.g. +hv-ipi,+hv-tlbflush,+hv-vpindex,+hv-reenlightenment advertises
a bunch of things, but not the HYPERCALL MSR.

That results in the guest identifying Hyper-V and setting a variety of PV
ops that then get ignored because hyperv_init() silently disables Hyper-V
for all intents and purposes.  The VMM (or its controller) is obviously
off in the weeds, but ideally the guest kernel would acknowledge the bad
setup in some way.

v2:
  - Add Vitaly's review.
  - Rebase to hyperv-next, commit 285f68afa8b2 ("x86/hyperv: Protect
    set_hv_tscchange_cb() against getting preempted"). [Vitaly]
  - Tweak the changelog in patch 01 to omit the example about a bad VM
    config since the NULL check is needed even if that specific issue is
    resolved.

v1: https://lore.kernel.org/all/20211028222148.2924457-1-seanjc@google.com/t/#u

Sean Christopherson (2):
  x86/hyperv: Fix NULL deref in set_hv_tscchange_cb() if Hyper-V setup
    fails
  x86/hyperv: Move required MSRs check to initial platform probing

 arch/x86/hyperv/hv_init.c      | 12 ++++--------
 arch/x86/kernel/cpu/mshyperv.c | 20 +++++++++++++++-----
 2 files changed, 19 insertions(+), 13 deletions(-)

-- 
2.34.0.rc0.344.g81b53c2807-goog