Re: [PATCH 00/21] KVM: x86: Event/exception fixes and cleanups

Maxim Levitsky
2022-03-13T09:22:56Z

On Fri, 2022-03-11 at 03:27 +0000, Sean Christopherson wrote:
> The main goal of this series is to fix KVM's longstanding bug of not
> honoring L1's exception intercepts wants when handling an exception that
> occurs during delivery of a different exception. E.g. if L0 and L1 are
> using shadow paging, and L2 hits a #PF, and then hits another #PF while
> vectoring the first #PF due to _L1_ not having a shadow page for the IDT,
> KVM needs to check L1's intercepts before morphing the #PF => #PF => #DF
> so that the #PF is routed to L1, not injected into L2 as a #DF.
>
> nVMX has hacked around the bug for years by overriding the #PF injector
> for shadow paging to go straight to VM-Exit, and nSVM has started doing
> the same. The hacks mostly work, but they're incomplete, confusing, and
> lead to other hacky code, e.g. bailing from the emulator because #PF
> injection forced a VM-Exit and suddenly KVM is back in L1.
>
> Everything leading up to that are related fixes and cleanups I encountered
> along the way; some through code inspection, some through tests (I truly
> thought this series was finished 10 commits and 3 days ago...).
>
> Nothing in here is all that urgent; all bugs tagged for stable have been
> around for multiple releases (years in most cases).
>
> Sean Christopherson (21):
> KVM: x86: Return immediately from x86_emulate_instruction() on code
> #DB
> KVM: nVMX: Unconditionally purge queued/injected events on nested
> "exit"
> KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS
> KVM: x86: Don't check for code breakpoints when emulating on exception
> KVM: nVMX: Treat General Detect #DB (DR7.GD=1) as fault-like
> KVM: nVMX: Prioritize TSS T-flag #DBs over Monitor Trap Flag
> KVM: x86: Treat #DBs from the emulator as fault-like (code and
> DR7.GD=1)
> KVM: x86: Use DR7_GD macro instead of open coding check in emulator
> KVM: nVMX: Ignore SIPI that arrives in L2 when vCPU is not in WFS
> KVM: nVMX: Unconditionally clear mtf_pending on nested VM-Exit
> KVM: VMX: Inject #PF on ENCLS as "emulated" #PF
> KVM: x86: Rename kvm_x86_ops.queue_exception to inject_exception
> KVM: x86: Make kvm_queued_exception a properly named, visible struct
> KVM: x86: Formalize blocking of nested pending exceptions
> KVM: x86: Use kvm_queue_exception_e() to queue #DF
> KVM: x86: Hoist nested event checks above event injection logic
> KVM: x86: Evaluate ability to inject SMI/NMI/IRQ after potential
> VM-Exit
> KVM: x86: Morph pending exceptions to pending VM-Exits at queue time
> KVM: VMX: Update MTF and ICEBP comments to document KVM's subtle
> behavior
> KVM: selftests: Use uapi header to get VMX and SVM exit reasons/codes
> KVM: selftests: Add an x86-only test to verify nested exception
> queueing
>
> arch/x86/include/asm/kvm-x86-ops.h | 2 +-
> arch/x86/include/asm/kvm_host.h | 33 +-
> arch/x86/kvm/emulate.c | 3 +-
> arch/x86/kvm/svm/nested.c | 100 ++---
> arch/x86/kvm/svm/svm.c | 18 +-
> arch/x86/kvm/vmx/nested.c | 322 +++++++++-----
> arch/x86/kvm/vmx/sgx.c | 2 +-
> arch/x86/kvm/vmx/vmx.c | 53 ++-
> arch/x86/kvm/x86.c | 409 ++++++++++++------
> arch/x86/kvm/x86.h | 10 +-
> tools/testing/selftests/kvm/.gitignore | 1 +
> tools/testing/selftests/kvm/Makefile | 1 +
> .../selftests/kvm/include/x86_64/svm_util.h | 5 +-
> .../selftests/kvm/include/x86_64/vmx.h | 51 +--
> .../kvm/x86_64/nested_exceptions_test.c | 307 +++++++++++++
> 15 files changed, 914 insertions(+), 403 deletions(-)
> create mode 100644 tools/testing/selftests/kvm/x86_64/nested_exceptions_test.c
>
>
> base-commit: 4a204f7895878363ca8211f50ec610408c8c70aa

I am just curious. Are you aware that I worked on this few months ago?
I am sure that you even reviewed some of my code back then.

If so, could you have had at least mentioned this and/or pinged me to continue
working on this instead of re-implementing it?

Best regards,
Maxim Levitsky