[Qemu-devel] [PATCH for-2.0?] target-i386: fix gdb debugging with large memory guests

[Luiz Capitulino <lcapitulino@redhat.com>]

If you start a Linux guest with more than 4GB of memory and try to look at a
memory address, you will get an error from gdb:

(gdb) p node_data[0]->node_id
Cannot access memory at address 0xffff88013fffd3a0
(gdb)

I debugged this down to x86_cpu_get_phys_page_debug(), it doesn't handle the
case where the PDPTE has the PS bit set (although I didn't check where Linux
sets that bit). This commit adds the PS bit handling, which fixes the problem
for me.

Signed-off-by: Luiz capitulino <lcapitulino@redhat.com>
---

Two observations:

 1. This bug has always existed, so it's not a regression, so I'm not sure
    it's worth it to fix for 2.0

 2. I'm not familiar with every detail of x86_cpu_get_phys_page_debug(),
    so I'm not completely sure this is the right thing to do

 target-i386/helper.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/target-i386/helper.c b/target-i386/helper.c
index 4f447b8..9b7803f 100644
--- a/target-i386/helper.c
+++ b/target-i386/helper.c
@@ -951,6 +951,13 @@ hwaddr x86_cpu_get_phys_page_debug(CPUState *cs, vaddr addr)
                 return -1;
         }
 
+        if (pdpe & PG_PSE_MASK) {
+            page_size = 1024 * 1024 * 1024;
+            pte = pdpe & ~( (page_size - 1) & ~0xfff);
+            pte &= ~(PG_NX_MASK | PG_HI_USER_MASK);
+            goto out;
+        }
+
         pde_addr = ((pdpe & ~0xfff & ~(PG_NX_MASK | PG_HI_USER_MASK)) +
                     (((addr >> 21) & 0x1ff) << 3)) & env->a20_mask;
         pde = ldq_phys(cs->as, pde_addr);
@@ -993,6 +1000,7 @@ hwaddr x86_cpu_get_phys_page_debug(CPUState *cs, vaddr addr)
         pte = pte & env->a20_mask;
     }
 
+out:
     page_offset = (addr & TARGET_PAGE_MASK) & (page_size - 1);
     paddr = (pte & TARGET_PAGE_MASK) + page_offset;
     return paddr;
-- 
1.8.3.1