[PATCH 6/7] x86/vga: fix mapping of the VGA text buffer

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The call to ioremap_wc() in video_init() will always fail, because
video_init() is called ahead of vm_init_type(), and so the underlying
__vmap() call will fail to allocate the linear address space.

Fix by reverting to the previous behavior and using the directmap entries
in the low 1MB.  Note the VGA text buffer directmap entries are also
adjusted to map the VGA text buffer as WC instead of UC-.

Fixes: 81d195c6c0e2 ('x86: introduce ioremap_wc()')
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/boot/x86_64.S        | 10 +++++++---
 xen/arch/x86/include/asm/config.h |  5 +++++
 xen/drivers/video/vga.c           | 11 ++++++++---
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index 26b9d1c2df9a..07f4bdf46e31 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -84,15 +84,19 @@ ENTRY(__high_start)
 /*
  * Mapping of first 2 megabytes of memory. This is mapped with 4kB mappings
  * to avoid type conflicts with fixed-range MTRRs covering the lowest megabyte
- * of physical memory. In any case the VGA hole should be mapped with type UC.
+ * of physical memory. VGA hole should be mapped with type UC, with the
+ * exception of the text buffer that uses WC.
  * Uses 1x 4k page.
  */
 l1_directmap:
         pfn = 0
         .rept L1_PAGETABLE_ENTRIES
-        /* VGA hole (0xa0000-0xc0000) should be mapped UC-. */
-        .if pfn >= 0xa0 && pfn < 0xc0
+        /* VGA hole (0xa0000-0xb8000) should be mapped UC-. */
+        .if pfn >= 0xa0 && pfn < 0xb8
         .quad (pfn << PAGE_SHIFT) | __PAGE_HYPERVISOR_UCMINUS | _PAGE_GLOBAL | 
MAP_SMALL_PAGES
+        /* VGA text buffer (0xb80000-0xc0000) should be mapped WC. */
+        .elseif pfn >= 0xb8 && pfn < 0xc0
+        .quad (pfn << PAGE_SHIFT) | __PAGE_HYPERVISOR_WC | _PAGE_GLOBAL | 
MAP_SMALL_PAGES
         .else
         .quad (pfn << PAGE_SHIFT) | PAGE_HYPERVISOR_RWX | MAP_SMALL_PAGES
         .endif
diff --git a/xen/arch/x86/include/asm/config.h 
b/xen/arch/x86/include/asm/config.h
index 19746f956ec3..a455bfb0df65 100644
--- a/xen/arch/x86/include/asm/config.h
+++ b/xen/arch/x86/include/asm/config.h
@@ -208,6 +208,11 @@
 #endif
 #define DIRECTMAP_VIRT_END      (DIRECTMAP_VIRT_START + DIRECTMAP_SIZE)
 
+#define IS_DIRECTMAP_ADDR(x) ({                                         \
+    unsigned long addr = (unsigned long)(x);                            \
+    addr >= DIRECTMAP_VIRT_START && addr < DIRECTMAP_VIRT_END;          \
+})
+
 #ifndef __ASSEMBLY__
 
 #ifdef CONFIG_PV32
diff --git a/xen/drivers/video/vga.c b/xen/drivers/video/vga.c
index b4d018326128..704d00034658 100644
--- a/xen/drivers/video/vga.c
+++ b/xen/drivers/video/vga.c
@@ -70,9 +70,13 @@ void __init video_init(void)
     switch ( vga_console_info.video_type )
     {
     case XEN_VGATYPE_TEXT_MODE_3:
-        if ( page_is_ram_type(paddr_to_pfn(0xB8000), RAM_TYPE_CONVENTIONAL) ||
-             ((video = ioremap_wc(0xB8000, 0x8000)) == NULL) )
+        if ( page_is_ram_type(paddr_to_pfn(0xB8000), RAM_TYPE_CONVENTIONAL) )
             return;
+        /*
+         * The first MB is always mapped, and the VGA text buffer uses the WC
+         * cache attribute.
+         */
+        video = __va(0xB8000);
         outw(0x200a, 0x3d4); /* disable cursor */
         columns = vga_console_info.u.text_mode_3.columns;
         lines   = vga_console_info.u.text_mode_3.rows;
@@ -158,7 +162,8 @@ void __init video_endboot(void)
         if ( !vgacon_keep )
         {
             memset(video, 0, columns * lines * 2);
-            iounmap(video);
+            /* VGA text buffer uses a directmap mapping, don't try to unmap. */
+            ASSERT(IS_DIRECTMAP_ADDR(video));
             video = ZERO_BLOCK_PTR;
         }
         break;
-- 
2.48.1