Re: Dealing with the NMI mess

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/07/2015 23:08, Andy Lutomirski wrote:
> user_icebp is set if int $0x01 happens, except it isn't because user
> code can't actually do that -- it'll cause #GP instead.
> 
> user_icebp is also set if the user has a bloody in-circuit emulator,
> given the name.  But who on Earth has one of those on a system new
> enough to run Linux and, even if they have one, why on Earth are they
> using it to send SIGTRAP.

You do not need either "int $0x01" or an ICE to set user_icebp = 1.  You
can use the 0xf1 opcode, which is kinda like 0xcc but generates #DB
instead of #BP.

The historical name is ICEBP because in-circuit emulators used it for
software breakpoints, just like your usual debugger used 0xcc aka int3.
 And just like 0xcc it's unprivileged, so you can actually get a SIGTRAP
with asm(".byte 0xf1").

So...

> In any event, user_icebp is only set if user_mode(regs), so it's safe
> locking-wise.  But please let's delete it.

... it's safe, but it has some use (!).

Paolo