[Xen-devel] [PATCHv2 2/3] x86: tss: Eliminate fragile calculation of TSS segment limit

[Josh Triplett <josh@xxxxxxxxxxxxxxxx>]

__set_tss_desc has a complex calculation of the TSS segment limit,
duplicating the quirky details of the I/O bitmap array length, and
requiring a complex comment to explain.  Replace that calculation with a
simpler one based on the offsetof the "stack" field that follows the
array.

That then removes the last use of IO_BITMAP_OFFSET, so delete it.

Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Acked-by: Alexander van Heukelum <heukelum@xxxxxxxxxxx>
---
v2: Rebased.  Added a comment explaining the -1 in the calculation.

 arch/x86/include/asm/desc.h      | 11 +----------
 arch/x86/include/asm/processor.h |  4 +++-
 2 files changed, 4 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 50d033a..f8dc455 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -177,16 +177,7 @@ static inline void __set_tss_desc(unsigned cpu, unsigned 
int entry, void *addr)
        struct desc_struct *d = get_cpu_gdt_table(cpu);
        tss_desc tss;
 
-       /*
-        * sizeof(unsigned long) coming from an extra "long" at the end
-        * of the iobitmap. See tss_struct definition in processor.h
-        *
-        * -1? seg base+limit should be pointing to the address of the
-        * last valid byte
-        */
-       set_tssldt_descriptor(&tss, (unsigned long)addr, DESC_TSS,
-                             IO_BITMAP_OFFSET + IO_BITMAP_BYTES +
-                             sizeof(unsigned long) - 1);
+       set_tssldt_descriptor(&tss, (unsigned long)addr, DESC_TSS, TSS_LIMIT);
        write_gdt_entry(d, entry, &tss, DESC_TSS);
 }
 
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index fdedd38..7da8794 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -259,9 +259,11 @@ struct x86_hw_tss {
 #define IO_BITMAP_BITS                 65536
 #define IO_BITMAP_BYTES                        (IO_BITMAP_BITS/8)
 #define IO_BITMAP_LONGS                        (IO_BITMAP_BYTES/sizeof(long))
-#define IO_BITMAP_OFFSET               offsetof(struct tss_struct, io_bitmap)
 #define INVALID_IO_BITMAP_OFFSET       0x8000
 
+/* Segment limits point to the last valid byte, hence the -1 */
+#define TSS_LIMIT      (offsetof(struct tss_struct, stack) - 1)
+
 struct tss_struct {
        /*
         * The hardware state:
-- 
1.9.0


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