Re: [Xen-devel] [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

[Borislav Petkov <bp@xxxxxxxxx>]

On Mon, Nov 16, 2015 at 12:11:11PM -0800, Andy Lutomirski wrote:
> On Mon, Nov 16, 2015 at 11:59 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> > On Mon, Nov 16, 2015 at 11:03:22AM -0800, Andy Lutomirski wrote:
> >
> >> ...
> >> The reader surely doesn't remember that this isn't guaranteed to be a
> >> swapgs instruction on native.  Using:
> >>
> >> ALTERNATIVE "swapgs" "" X86_FEATURE_XENPV
> >>
> >> would be safer (it would get rid of the SWAPGS_UNSAFE_STACK mess) and
> >> much clearer.  We could hide *that* behind a macro and no one would be
> >> confused.  (Well, they'd be confused by the fact that Xen PV handles
> >> gsbase very differently from native, but that has nothing to do with
> >> the macro.)
> >>
> >> I think we could convert piecemeal, and I wonder if this new patch for
> >> 32-bit native on 4.4 (this is needed for 4.4, right?) would be a good
> >> starting point.  Borislav, what do you think?  Would you be okay with
> >> adding a Xen PV pseudofeature?
> >
> > AFAICT, I'd prefer this becomes rather a jump label which gets enabled
> > on xen. Especially if a single pseudofeature might not be enough,
> > apprently...
> 
> Except it's not a jump.  (Also, the alternatives infrastructure is IMO
> much nicer than the jump label infrastructure.)
> 
> Taking SWAPGS as an example, the semantics we need are:
> 
>  - On native, do swapgs.  This *can't* be a call due to RSP issues.
>  - On Xen PV, swapgs will work, but it's emulated.  We'd rather just nop it 
> out.

Huh, so what's wrong with a jump:

        jmp 1f
        swapgs
        1:

> In principle, we could static jump over it on Xen, but that also
> involves forcing the jump label to be built on old GCC versions, which
> PeterZ objected to the last time I asked.

:-\

> If it would make you feel better, it could be X86_BUG_XENPV :-p

That doesn't matter - I just don't want to open the flood gates on
pseudo feature bits.

hpa, what do you think?

> Are there really multiple feature bits for this stuff?  I'd like to
> imagine that the entry code is all either Xen PV or native/PVH/PVHVM
> -- i.e. I assumed that PVH works like native for all entries.

I just reacted to Boris' statement:

"We don't currently have a Xen-specific CPU feature. We could, in
principle, add it but we can't replace all of current paravirt patching
with a single feature since PVH guests use a subset of existing pv ops
(and in the future it may become even more fine-grained)."

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.

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