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

[Ricardo Neri]

On Thu, 2017-03-30 at 13:10 +0300, Stas Sergeev wrote:
> 30.03.2017 08:14, Ricardo Neri ÐÐÑÐÑ:
> >>>> But at least dosemu implements it, so probably it is needed.
> >>> Right.
> >>>
> >>>> 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. :)
> >>> Do you mean relaying the GP fault to dosemu instead of trapping it and
> >>> emulating it in the kernel?
> >> Yes, that would be optimal if this does not severely break
> >> the current setups. If we can find out that smsw is not in
> >> the real use, we can probably do exactly that.
> >> But other
> >> instructions are not in real use in v86 for sure, so I
> >> wouldn't be adding the explicit test-cases to the kernel
> >> that will make you depend on some particular behaviour
> >> that no one may need.
> >> My objection was that we shouldn't
> >> write tests before we know exactly how we want this to work.
> > OK, if only SMSW is used then I'll keep the emulation for SMSW only.
> In fact, smsw has an interesting property, which is that
> no one will ever want to disable its in-kernel emulation
> to provide its own.
> So while I'll try to estimate its usage, emulating it in kernel
> will not be that problematic in either case.

Ah good to know!

> As for protected mode, if wine only needs sgdt/sidt, then
> again, no one will want to disable its emulation. Not the
> case with sldt, but AFAICS wine doesn't need sldt, and so
> we can leave sldt without a fixups. Is my understanding
> correct?

This is my understanding as well. I could not find any use of sldt in
wine. Alexandre, would you mind confirming?

> In this case, I suppose, we are very well on a way to avoid
> the extra syscalls to toggle the emulation features.

Great! Then I will keep the emulation for sgdt, sidt, and smsw but not
for str and sldt; for both vm86 and protected mode. This seems to be the
agreement.

Thanks and BR,
Ricardo