Re: [Qemu-devel] [RFC PATCH] i386: Remove ospke CPUID flag name

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jun 13, 2018 at 07:01:23PM +0200, Paolo Bonzini wrote:
> On 12/06/2018 20:25, Eduardo Habkost wrote:
> > On Tue, Jun 12, 2018 at 05:12:58PM +0200, Paolo Bonzini wrote:
> >> On 12/06/2018 17:01, Eduardo Habkost wrote:
> >>>>>
> >>>>> Remove OSPKE from the feature name array so users don't try to
> >>>>> configure it manually.
> >>>>>
> >>>>> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> >>>> Yes, it's the same as OSXSAVE.  Thanks!
> >>> CR4_OSXSAVE_MASK is automatically enabled on user-mode QEMU,
> >>> though.
> >>>
> >>> My question is if it would make any sense to enable CR4_PKE_MASK
> >>> too.
> >>
> >> If you mean OSPKE, then yes---if PKU is available.  Likewise, OSXSAVE
> >> should only be enabled if XSAVE is available.
> > 
> > Yeah, I mean enabling it only if PKU is available, like we
> > already do with OSXAVE/XSAVE.
> > 
> > But we don't do it today, so enabling it automatically in
> > CONFIG_USER_ONLY would be a new feature.  Would it be useful for
> > anything, though?
> > 
> > I'm asking that to find out if somebody could be already using
> > "-cpu ...,+ospke" with user-mode QEMU today (which this patch
> > would break).  If RDPKRU/WRPKRU is useless under user-mode QEMU,
> > than we don't need to worry about that.
> 
> Hmm, actually there are two more things to consider for user-mode emulation.
> 
> First, QEMU doesn't support any of pkey_mprotect, pkey_alloc, pkey_free,
> so it should probably never set OSPKE.

This means "-cpu ...,+ospke" is useless today and we can safely
remove support for it, right?

> Second, for user-mode emulation it makes sense to allow flipping of
> OSPKE and OSXSAVE, because that corresponds to different behaviors of
> the underlying kernels.  There have been bugs in fact with programs that
> incorrectly tested XSAVE instead of OSXSAVE, so it's worthwhile to let
> the user test both configurations.

However, "-cpu ...,-osxsave" has no effect today (user-mode QEMU
unconditionally sets OSXSAVE), so that would be a new feature.

I assume this means I don't need to drop the osxsave patch from
my queue, either.

> So to sum up, the default for QEMU user-mode emulation should be
> OSXSAVE=XSAVE and OSPKE=0.

Thanks!

-- 
Eduardo