getting a 32-bit mfn from a 32-on-64 HVM guest

[Dan Magenheimer <dan.magenheimer@oracle.com>]

I'm working on getting tmem working for HVM guests
(on top of Stefano's PV on HVM patch).  I've discovered
that the guest mfn passed in for some tmem operations
from a 32-bit HVM guest is getting interpreted in
the following function as a 64-bit mfn, and the
upper bits are confusing the translation to a Xen mfn.

What is the "proper" way to ensure that the cmfn
is properly truncated for a 32-bit HVM guest
without truncating it for a 64-bit guest?  I
have used is_pv_32on64_vcpu()... is there an equivalent
for HVM? Or do I need to do something entirely different?

Thanks,
Dan

/* from xen/common/tmem_xen.c */
static inline void *cli_mfn_to_va(tmem_cli_mfn_t cmfn,
   unsigned long *pcli_mfn)
{
    unsigned long cli_mfn;
    p2m_type_t t;

    cli_mfn = mfn_x(gfn_to_mfn(current->domain, cmfn, &t));
    if (t != p2m_ram_rw)
        return NULL;
    if (pcli_mfn != NULL)
        *pcli_mfn = cli_mfn;
    return map_domain_page(cli_mfn);
}

/* following from include/public/tmem.h, look for cmfn */
typedef xen_pfn_t tmem_cli_mfn_t;
typedef XEN_GUEST_HANDLE(char) tmem_cli_va_t;
struct tmem_op {
    uint32_t cmd;
    int32_t pool_id;
    union {
        struct {
            uint64_t uuid[2];
            uint32_t flags;
            uint32_t arg1;
        } new; /* for cmd == TMEM_NEW_POOL, TMEM_AUTH, TMEM_RESTORE_NEW */
        struct { 
            uint32_t subop;
            uint32_t cli_id;
            uint32_t arg1;
            uint32_t arg2;
            uint64_t arg3;
            tmem_cli_va_t buf;
        } ctrl; /* for cmd == TMEM_CONTROL */
        struct {
            
            uint64_t object;
            uint32_t index;
            uint32_t tmem_offset;
            uint32_t pfn_offset;
            uint32_t len;
            tmem_cli_mfn_t cmfn; /* client machine page frame */
        } gen; /* for all other cmd ("generic") */
    } u;
};
typedef struct tmem_op tmem_op_t;
DEFINE_XEN_GUEST_HANDLE(tmem_op_t);