Re: xl/xm save -c fails - set_vcpucontext EOPNOTSUPP (was Re: xl save -c issues with Windows 7 Ultimate)

[Shriram Rajagopalan <rshriram@cs.ubc.ca>]

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On Tue, May 10, 2011 at 10:02 AM, Jan Beulich <JBeulich@novell.com> wrote:

> >>> On 10.05.11 at 16:52, Shriram Rajagopalan <rshriram@cs.ubc.ca> wrote:
> > On Tue, May 10, 2011 at 3:41 AM, Ian Campbell <Ian.Campbell@citrix.com
> >wrote:
> >> The most plausible looking EOPNOTSUPP from that code is in
> >> xen/arch/x86/domain.c:arch_set_info_guest() but that is on a PV only
> >> path.
> >>
> >> And that rings with the pv guests I am using. It makes perfect sense,
> > looking
> > at that function and especially at the code that returns EOPNOTSUPP (the
> > only
> > place in the entire file).
> >    else
> >     {
> >         bool_t fail = v->arch.pv_vcpu.ctrlreg[3] != c(ctrlreg[3]);
> >
> > #ifdef CONFIG_X86_64
> >         fail |= v->arch.pv_vcpu.ctrlreg[1] != c(ctrlreg[1]);
> > #endif
> >
> >         for ( i = 0; i < ARRAY_SIZE(v->arch.pv_vcpu.gdt_frames); ++i )
> >             fail |= v->arch.pv_vcpu.gdt_frames[i] != c(gdt_frames[i]);
> >         fail |= v->arch.pv_vcpu.gdt_ents != c(gdt_ents);
> >
> >         fail |= v->arch.pv_vcpu.ldt_base != c(ldt_base);
> >         fail |= v->arch.pv_vcpu.ldt_ents != c(ldt_ents);
> >
> >         if ( fail )
> >            return -EOPNOTSUPP;
> >     }
> >
> > This change was introduced by c/s
> > changeset:   23142:f5e8d152a565
> > user:        Jan Beulich <jbeulich@novell.com>
> > date:        Tue Apr 05 13:01:25 2011 +0100
> > x86: split struct vcpu
> >
> > I think I am missing something really obvious in this piece of code. The
> > xc_domain_resume code tries to modify the return value of shutdown
> hypercall
> > (i.e eax register is set to 1) and this code doesnt seem to check those
> > registers.
>
> Correct - the code here checks only for values where the logic
> needed to support changing the on an already initialized vCPU isn't
> implemented. Previously, actual vCPU state and what was tracked
> in struct vcpu could get out of sync in this case, potentially
> confusing things further down (including possible security issues).
>
> You'll want to figure out which part(s) actually differ, and why.
> Only then we'll be able to tell whether mentioned c/s introduced
> false positives.
>
> Bit confused. If I understand correctly, this piece of code checks new
values
of certain registers against old ones, for an already initialized VCPU. And
AFAIT,
it is checking the gdts, ldts & control registers. The xc_domain_resume code
just
changes one general purpose register eax. basically,

 get_vcpucontext()
 set_field(eax, 1) //to indicate SUSPEND_CANCEL
 set_vcpucontext()

I dont understand what you mean by "which parts actually differ & why".


And just a trivial question:
 is the hypervisor binary always compiled to a 32-bit elf? somehow, the
symbols file xen-syms-* is getting compiled to 64 bit ELF binary while
the xen binary is getting compiled to 32-bit binary.

shriram

> Jan
>
>

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