Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

[Stas Sergeev]

28.03.2017 02:46, Ricardo Neri ÐÐÑÐÑ:
> On Tue, 2017-03-14 at 00:25 +0300, Stas Sergeev wrote:
> > 11.03.2017 02:59, Ricardo Neri ÐÐÑÐÑ:
> > > On Fri, 2017-03-10 at 14:33 +0300, Stas Sergeev wrote:
> > >
> > > > Why would you need one?
> > > > Or do you really want to allow these instructions
> > > > in v86 by the means of emulation? If so - this wasn't
> > > > clearly stated in the patch description, neither it was
> > > > properly discussed, it seems.
> > > It str and sldt can be emulated in vm86 but as Andy mention, the
> > > behavior sould be the same with and without emulation.
> > Why would you do that?
> > I looked up the dosemu2 CPU simulator code that
> > is used under x86-64. It says this:
> Stas, I apologize for the delayed reply; I missed your e-mail.
> > It only implements smsw.
> > So maybe you can make your code much
> > simpler and remove the unneeded emulation?
> > Same is for prot mode.
> Do you mean the unneeded emulation for SLDT and STR?
Not quite, I meant also sgdt and sidt in vm86.
Yes that it will be a somewhat "incompatible" change,
but if there is nothing to stay compatible with,
then why to worry? Probably you could also remove
the sldt and str emulation for protected mode, because,
as I understand from this thread, wine does not
need those.

Note that these days dosemu2 uses v86 mode set
up under kvm rather than vm86(). Your patches
affect that the same way as they do for vm86()
syscall, or can there be some differences? Or should
the UMIP be enabled under kvm by hands?

> > You know the wine's
> > requirements now - they are very small. And
> > dosemu doesn't need anything at all but smsw.
> > And even smsw is very rare.
> But emulation is still needed for SMSW, right?
Likely so.
If you want, I can enable the logging of this command
and see if it is used by some of the DOS programs I have.
But at least dosemu implements it, so probably it is needed.
Of course if it is used by one of 100 DOS progs, then there
is an option to just add its support to dosemu2 and pretend
the compatibility problems did not exist. :) So, if this can be
an option, I can do the tests to estimate its usage.