x86/pv32: Fix bogus CR2 in pagefault for gate emulation

__{put,get}_guest() returns -EFAULT on access faults which causes
the injected CR2 to be off by 14 bytes (as EFAULT is 14) which is
incorrect.

Fix the computation by relying on __copy_{from,to}_guest_pv() which
reports the number of remaining bytes instead of a negative errno,
such that we can compute the offset properly.

Fixes: 70ad570b27 ("x86/64: paravirt 32-on-64 call gate support")
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/pv/emul-gate-op.c

@@ -286,12 +286,14 @@ void pv_emulate_gate_op(struct cpu_user_regs *regs)
     if ( !jump )
     {
         unsigned int ss, esp, *stkp;
+        uint32_t value;
         int rc;
 #define push(item) do \
         { \
+            value = (item); \
             --stkp; \
             esp -= 4; \
-            rc = __put_guest(item, stkp); \
+            rc = __copy_to_guest_pv(stkp, &value, sizeof(value)); \
             if ( rc ) \
             { \
                 pv_inject_page_fault(PFEC_write_access, \
@@ -359,7 +361,7 @@ void pv_emulate_gate_op(struct cpu_user_regs *regs)
                     unsigned int parm;
 
                     --ustkp;
-                    rc = __get_guest(parm, ustkp);
+                    rc = __copy_from_guest_pv(&parm, ustkp, sizeof(parm));
                     if ( rc )
                     {
                         pv_inject_page_fault(0, (unsigned long)(ustkp + 1) - rc);