Re: [Xen-devel] [PATCH v4 8/8] x86/vm_event: Add HVM debug exception vm_events

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

>>> @@ -3393,8 +3409,9 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
>>>              }
>>>              else {
>>>                  int handled =
>>> -                    hvm_monitor_breakpoint(regs->eip,
>>> -                                           
>>> HVM_MONITOR_SOFTWARE_BREAKPOINT);
>>> +                        hvm_monitor_debug(regs->eip,
>>> +                                          HVM_MONITOR_SOFTWARE_BREAKPOINT,
>>> +                                          X86_EVENTTYPE_SW_EXCEPTION, 1);
>> Please let's not add further mistakes like this, assuming INT3 can't
>> have any prefixes. It can, even if they're useless.
> You mean the instruction length is not necessarily 1? Ultimately it
> doesn't seem to matter because reinjecting it with xc_hvm_inject_trap
> ignores this field. Instruction length is only required to be properly
> set AFAICT for a subset of debug exceptions during reinjection.

Almost all x86 instructions can have redundant prefixes which change
their length without altering their functionality.

This specific area was the subject of XSA-106, and is astoundingly fragile.

Luckily, I have an available functional test which confirms correct
behaviour from the point of view of the guest.

http://xenbits.xen.org/people/andrewcoop/xen-test-framework/test-swint-emulation.html

Please confirm that this test returns success even when being monitored
with this new functionality.

~Andrew

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