RE: [Xen-devel] testing hypercall from windows - what's the most basic call I can make to test

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> As for the simplest hypercall, that is xen_version.
> 

Am I correct in saying that 'HYPERVISOR_xen_version(0, NULL)' is a
benign call, and that if the system doesn't crash, I'm probably on the
right track? 

So far, I get a STOP error 0x1000007e (0xC0000005, ...) which is a
'SYSTEM THREAD EXCEPTION NOT HANDLED' error, with the exception being
'ACCESS VIOLATION'.

Xen_version appears to use the two parameter version of the hypercall...
in the linux header this is defined as:

"
#define _hypercall2(type, name, a1, a2)                         \
({                                                              \
        long __res, __ign1, __ign2;                             \
        asm volatile (                                          \
                HYPERCALL_STR(name)                             \
                : "=a" (__res), "=b" (__ign1), "=c" (__ign2)    \
                : "1" ((long)(a1)), "2" ((long)(a2))            \
                : "memory" );                                   \
        (type)__res;                                            \
})
"

Now the only x86 assembler I have ever done was at Uni, just over 10
years ago, so I'm guessing the probably is probably my translation to
Microsoft (Intel) compatible inline assembly here:

"
static __forceinline int
HYPERVISOR_xen_version(int cmd, void *arg)
{
  long __res;
  __asm {
    push ebp
    mov ebp, esp
    sub esp, 8
    mov eax, cmd
    mov [ebp - 8], eax
    mov eax, arg
    mov [ebp - 4], eax
    call hypercall_stubs + (__HYPERVISOR_xen_version * 32)
    add esp, 8
    pop ebp
    mov [__res], eax
  }
  return __res;
}
"

The compiler complains that I'm tinkering with ebp, but I am putting it
back afterwards so I don't see that as a problem.

When I allocate the memory for 'hypercall_stubs', I check the first two
bytes at the (__HYPERVISOR_xen_version * 32) offset before and after the
WRMSR call, and they definitely do change, so I believe the WRMSR call
is correct.

Any suggestions? Writing this email is mostly just therapeutic... so
many times I'll fuss over a problem for ages, finally send a request for
help to a mailing list, and find the problem almost immediately after,
so I'm hoping this will happen here :)

Thanks

James

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