[xen staging] x86/pv: Rename pv.iobmp_limit to iobmp_nr and clarify behaviour

[patchbot@xxxxxxx]

commit 633ee8b2df963f7e5cb8de1219c1a48bfb4447f6
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Oct 1 13:00:13 2024 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Oct 1 14:58:18 2024 +0100

    x86/pv: Rename pv.iobmp_limit to iobmp_nr and clarify behaviour
    
    Ever since it's introduction in commit 013351bd7ab3 ("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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/include/asm/domain.h | 2 +-
 xen/arch/x86/physdev.c            | 2 +-
 xen/arch/x86/pv/emul-priv-op.c    | 6 +++++-
 xen/include/public/physdev.h      | 3 +++
 4 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/include/asm/domain.h 
b/xen/arch/x86/include/asm/domain.h
index 811251852f..bdcdb8de09 100644
--- a/xen/arch/x86/include/asm/domain.h
+++ b/xen/arch/x86/include/asm/domain.h
@@ -573,7 +573,7 @@ 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. */
diff --git a/xen/arch/x86/physdev.c b/xen/arch/x86/physdev.c
index d6dd622952..69fd42667c 100644
--- a/xen/arch/x86/physdev.c
+++ b/xen/arch/x86/physdev.c
@@ -436,7 +436,7 @@ 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;
     }
 
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index e35285d4ab..70150c2722 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/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;
diff --git a/xen/include/public/physdev.h b/xen/include/public/physdev.h
index 45e1c18541..3149049a9a 100644
--- a/xen/include/public/physdev.h
+++ b/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 {
--
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