x86/pv: Rename pv.iobmp_limit to iobmp_nr and clarify behaviour

Ever since it's introduction in commit 013351bd7a ("Define new event-channel
and physdev hypercalls") in 2006, the public interface was named nr_ports
while the internal field was called iobmp_limit.

Rename the internal field to iobmp_nr to match the public interface, and
clarify that, when nonzero, Xen will read 2 bytes.

There isn't a perfect parallel with a real TSS, but iobmp_nr being 0 is the
paravirt "no IOPB" case, and it is important that no read occurs in this case.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 633ee8b2df
master date: 2024-10-01 14:58:18 +0100

xen/arch/x86/include/asm/domain.h

@@ -574,7 +574,7 @@ struct pv_vcpu
 
     /* I/O-port access bitmap. */
     XEN_GUEST_HANDLE(uint8) iobmp; /* Guest kernel vaddr of the bitmap. */
-    unsigned int iobmp_limit; /* Number of ports represented in the bitmap. */
+    unsigned int iobmp_nr;    /* Number of ports represented in the bitmap. */
 #define IOPL(val) MASK_INSR(val, X86_EFLAGS_IOPL)
     unsigned int iopl;        /* Current IOPL for this VCPU, shifted left by
                                * 12 to match the eflags register. */

xen/arch/x86/physdev.c

@@ -433,7 +433,7 @@ ret_t do_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 #else
         guest_from_compat_handle(curr->arch.pv.iobmp, set_iobitmap.bitmap);
 #endif
-        curr->arch.pv.iobmp_limit = set_iobitmap.nr_ports;
+        curr->arch.pv.iobmp_nr = set_iobitmap.nr_ports;
         break;
     }
 

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

@@ -167,7 +167,11 @@ static int guest_io_okay(unsigned int port, unsigned int bytes,
     if ( iopl_ok(v, regs) )
         return X86EMUL_OKAY;
 
-    if ( (port + bytes) <= v->arch.pv.iobmp_limit )
+    /*
+     * When @iobmp_nr is non-zero, Xen, like real CPUs and the TSS IOPB,
+     * always reads 2 bytes from @iobmp, which might be one byte @iobmp_nr.
+     */
+    if ( (port + bytes) <= v->arch.pv.iobmp_nr )
     {
         const void *__user addr = v->arch.pv.iobmp.p + (port >> 3);
         uint16_t mask;

xen/include/public/physdev.h

@@ -87,6 +87,9 @@ DEFINE_XEN_GUEST_HANDLE(physdev_set_iopl_t);
 /*
  * Set the current VCPU's I/O-port permissions bitmap.
  * @arg == pointer to physdev_set_iobitmap structure.
+ *
+ * When @nr_ports is non-zero, Xen, like real CPUs and the TSS IOPB, always
+ * reads 2 bytes from @bitmap, which might be one byte beyond @nr_ports.
  */
 #define PHYSDEVOP_set_iobitmap           7
 struct physdev_set_iobitmap {