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

[Shriram Rajagopalan <rshriram@xxxxxxxxx>]

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

>>> On 10.05.11 at 16:52, Shriram Rajagopalan <rshriram@xxxxxxxxx> wrote:
> On Tue, May 10, 2011 at 3:41 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx>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@xxxxxxxxxx>
> 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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