Re: [Xen-devel] [PATCH][SVM] CPL initialization fix

Robert Phillips
Mon, 10 Mar 2008 20:01:55 -0000

Neither of the sections that you quote specify that it is correct to set the
CPL from the SS DPL.  They say (a) the SS DPL is set from the CPL and (b)
the SS DPL is (always mysteriously) equal to the CPL

I found the AMD (and Intel) manuals to be inconsistent.

AMD''s manual, Vol.2:
4.9.2 Privilege-Level Types
Current Privilege-Level. The current privilege-level (CPL) is the privilege
level at which the processor is currently executing. The CPL is stored in an
internal processor register that is invisible to software. Software changes
the CPL by performing a control transfer to a different code segment with a
new privilege level.

So the processor switches CPL by changing code segments, and the CPL is
taken from the code segment''s DPL.

And what, one might ask, if the stack segment and code segment have
different DPLs?  The AMD manual doesn''t say.  It simply asserts that
they
will be equal.

In our experience this is not true.

When we perform a live migrate from a HVM VS without PV drivers [what Ben
Guthro referred to as a QEMU-only virtual server], we simply pause the guest
in whatever state we find it.  We found that when the guest happened to be
in user-mode code (CPL == 3), the migrate failed because, on the receiving
node, the CPL would be restored to 0.  The zero came from SS.DPL.  However
the value in CS.DPL was 3.
SS.DPL was not equal to CS.DPL.
When we changed the code (per this patch), the CPL would be restored from
CS.DPL.  That value was correct in all cases.

-- rsp


On 3/10/08, Keir Fraser <keir.fraser@eu.citrix.com>
wrote:
>
> For reference, from AMD''s manual, Vol.2:
> "The processor reads the current privilege level from the CPL field in
the
> VMCB, not from SS.DPL. However, SS.DPL should match the CPL field."
>
> And from Intel''s manual, Vol.3B:
> "Note that the value of the DPL field for SS is always equal to the
> logical
> processor¹s current privilege level (CPL)."
>
>
>   -- Keir
>
>
> On 10/3/08 19:18, "Keir Fraser" <keir.fraser@eu.citrix.com>
wrote:
>
> > CPL is architecturally SS.DPL in almost all cases. It''s
certainly not
> > CS.DPL. What do you mean by ''migrate a QEMU guest''?
> >
> >  -- Keir
> >
> > On 10/3/08 19:14, "Ben Guthro"
<bguthro@virtualiron.com> wrote:
> >
> >> CPL should be determined by code segment''s DPL, not data
segment''s.
> >>
> >> This solves a problem migrating a QEMU guest without PV-on-HVM
drivers
> >>
> >> Signed-off-by: Robert Plillips <rphillips@virtualiron.com>
> >> Signed-off-by: Ben Guthro <bguthro@virtualoiron.com>
> >> diff -r f76f151a7431 xen/arch/x86/hvm/svm/svm.c
> >> --- a/xen/arch/x86/hvm/svm/svm.c
> >> +++ b/xen/arch/x86/hvm/svm/svm.c
> >> @@ -557,7 +557,7 @@ static void svm_set_segment_register(str
> >>          break;
> >>      case x86_seg_ss:
> >>          memcpy(&vmcb->ss, reg, sizeof(*reg));
> >> -        vmcb->cpl = vmcb->ss.attr.fields.dpl;
> >> +        vmcb->cpl = vmcb->cs.attr.fields.dpl;
> >>          break;
> >>      case x86_seg_tr:
> >>          memcpy(&vmcb->tr, reg, sizeof(*reg));
> >> _______________________________________________
> >> Xen-devel mailing list
> >> Xen-devel@lists.xensource.com
> >> http://lists.xensource.com/xen-devel
> >
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@lists.xensource.com
> > http://lists.xensource.com/xen-devel
>
>
>
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> http://lists.xensource.com/xen-devel
>



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Robert S. Phillips                          Virtual Iron Software
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