Re: [PATCH v2 00/21] KVM: x86: Event/exception fixes and cleanups

[Jim Mattson]

On Wed, Jun 29, 2022 at 4:17 AM Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote:
>
> On Tue, 2022-06-14 at 20:47 +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.
> >
> > v2:
> >   - Rebased to kvm/queue (commit 8baacf67c76c) + selftests CPUID
> >     overhaul.
> >     https://lore.kernel.org/all/20220614200707.3315957-1-seanjc@xxxxxxxxxx
> >   - Treat KVM_REQ_TRIPLE_FAULT as a pending exception.
> >
> > v1: https://lore.kernel.org/all/20220311032801.3467418-1-seanjc@xxxxxxxxxx
> >
> > Sean Christopherson (21):
> >   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: x86: Treat pending TRIPLE_FAULT requests as pending exceptions
> >   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               |  35 +-
> >  arch/x86/kvm/emulate.c                        |   3 +-
> >  arch/x86/kvm/svm/nested.c                     | 102 ++---
> >  arch/x86/kvm/svm/svm.c                        |  18 +-
> >  arch/x86/kvm/vmx/nested.c                     | 319 +++++++++-----
> >  arch/x86/kvm/vmx/sgx.c                        |   2 +-
> >  arch/x86/kvm/vmx/vmx.c                        |  53 ++-
> >  arch/x86/kvm/x86.c                            | 404 +++++++++++-------
> >  arch/x86/kvm/x86.h                            |  11 +-
> >  tools/testing/selftests/kvm/.gitignore        |   1 +
> >  tools/testing/selftests/kvm/Makefile          |   1 +
> >  .../selftests/kvm/include/x86_64/svm_util.h   |   7 +-
> >  .../selftests/kvm/include/x86_64/vmx.h        |  51 +--
> >  .../kvm/x86_64/nested_exceptions_test.c       | 295 +++++++++++++
> >  15 files changed, 886 insertions(+), 418 deletions(-)
> >  create mode 100644 tools/testing/selftests/kvm/x86_64/nested_exceptions_test.c
> >
> >
> > base-commit: 816967202161955f398ce379f9cbbedcb1eb03cb
>
> Hi Sean and everyone!
>
>
> Before I continue reviewing the patch series, I would like you to check if
> I understand the monitor trap/pending debug exception/event injection
> logic on VMX correctly. I was looking at the spec for several hours and I still have more
> questions that answers about it.
>
> So let me state what I understand:
>
> 1. Event injection (aka eventinj in SVM terms):
>
>   (VM_ENTRY_INTR_INFO_FIELD/VM_ENTRY_EXCEPTION_ERROR_CODE/VM_ENTRY_INSTRUCTION_LEN)
>
>   If I understand correctly all event injections types just like on SVM just inject,
>   and never create something pending, and/or drop the injection if event is not allowed
>   (like if EFLAGS.IF is 0). VMX might have some checks that could fail VM entry,
>   if for example you try to inject type 0 (hardware interrupt) and EFLAGS.IF is 0,
>   I haven't checked this)

The event is never just "dropped." If it is illegal to deliver the
event, VM-entry fails. See the second bullet under section 26.2.1.3:
VM-Entry Control Fields, in the SDM, volume 3.


>   All event injections happen right away, don't deliver any payload (like DR6), etc.

Correct.

>   Injection types 4/5/6, do the same as injection types 0/2/3 but in addition to that,
>   type 4/6 do a DPL check in IDT, and also these types can promote the RIP prior
>   to pushing it to the exception stack using VM_ENTRY_INSTRUCTION_LEN to be consistent
>   with cases when these trap like events are intercepted, where the interception happens
>   on the start of the instruction despite exceptions being trap-like.

Unlike the AMD "INTn intercept," these trap intercepts *do not* happen
at the start of the instruction. In early Intel VT-x parts, one could
not easily reinject an intercepted software interrupt or exception
using event injection, because VM-entry required a non-zero
instruction length, and the guest RIP had already advanced. On CPUs
that support a non-zero instruction length, one can now reinject a
software interrupt or exception, by setting the VM-entry instruction
length to 0.

> 2. #DB is the only trap like exception that can be pending for one more instruction
>    if MOV SS shadow is on (any other cases?).

I believe that's it. I'm not entirely sure about RTM,though.

>    (AMD just ignores the whole thing, rightfully)

When you say "ignores," do you mean that AMD ignores a data breakpoint
or single-step trap generated by MOV-SS, or it ignores the fact that
delivering such a #DB trap between the MOV-SS and the subsequent
MOV-ESP will create a stack frame in the wrong place?

>    That is why we have the GUEST_PENDING_DBG_EXCEPTIONS vmcs field.
>    I understand that it will be written by CPU in case we have VM exit at the moment
>    where #DB is already pending but not yet delivered.
>
>    That field can also be (sadly) used to "inject" #DB to the guest, if the hypervisor sets it,
>    and this #DB will actually update DR6 and such, and might be delayed/lost.

Injecting a #DB this way (if the hypervisor just emulated MOV-SS) is
easier than emulating the next instruction or using MTF to step
through the next instruction, and getting all of the deferred #DB
delivery rules right. :-)

>
> 3. Facts about MTF:
>
>    * MTF as a feature is basically 'single step the guest by generating MTF VM exits after each executed
>      instruction', and is enabled in primary execution controls.
>
>    * MTF is also an 'event', and it can be injected separately by the hypervisor with event type 7,
>      and that has no connection to the 'feature', although usually this injection will be useful
>      when the hypervisor does some kind of re-injection, triggered by the actual MTF feature.
>
>    * MTF event can be lost, if higher priority VM exit happens, this is why the SDM says about 'pending MTF',
>      which means that MTF vmexit should happen unless something else prevents it and/or higher priority VM exit
>      overrides it.

Hence, the facility for injecting a "pending MTF"--so that it won't be "lost."

>    * MTF event is raised (when the primary execution controls bit is enabled) when:
>
>         - after an injected (vectored), aka eventinj/VM_ENTRY_INTR_INFO_FIELD, done updating the guest state
>           (that is stack was switched, stuff was pushed to new exception stack, RIP updated to the handler)
>           I am not 100% sure about this but this seems to be what PRM implies:
>
>           "If the “monitor trap flag” VM-execution control is 1 and VM entry is injecting a vectored event (see Section
>           26.6.1), an MTF VM exit is pending on the instruction boundary before the first instruction following the
>           VM entry."
>
>         - If an interrupt and or #DB exception happens prior to executing first instruction of the guest,
>           then once again MTF will happen on first instruction of the exception/interrupt handler
>
>           "If the “monitor trap flag” VM-execution control is 1, VM entry is not injecting an event, and a pending event
>           (e.g., debug exception or interrupt) is delivered before an instruction can execute, an MTF VM exit is pending
>           on the instruction boundary following delivery of the event (or any nested exception)."
>
>           That means that #DB has higher priority that MTF, but not specified if fault DB or trap DB

These are single-step, I/O and data breakpoint traps.

>         - If instruction causes exception, once again, on first instruction of the exception handler MTF will happen.
>
>         - Otherwise after an instruction (or REP iteration) retires.
>
>
> If you have more facts about MTF and related stuff and/or if I made a mistake in the above, I am all ears to listen!

You might be interested in my augmented Table 6-2 (from volume 3 of
the SDM): https://docs.google.com/spreadsheets/d/e/2PACX-1vR8TkbSl4TqXtD62agRUs1QY3SY-98mKtOh-s8vYDzaDmDOcdfyTvlAxF9aVnHWRu7uyGhRwvHUziXT/pubhtml