[PATCH] target/i386/tcg: Fix masking of real-mode addresses with A20 bit

[PATCH] target/i386/tcg: Fix masking of real-mode addresses with A20 bit

[Stephen Michael Jothen]

The correct A20 masking is done if paging is enabled (protected mode) but it
seems to have been forgotten in real mode. For example from the AMD64 APM Vol. 2
section 1.2.4:

> If the sum of the segment base and effective address carries over into bit 20,
> that bit can be optionally truncated to mimic the 20-bit address wrapping of the
> 8086 processor by using the A20M# input signal to mask the A20 address bit.

Most BIOSes will enable the A20 line on boot, but I found by disabling the A20 line
afterwards, the correct wrapping wasn't taking place.

`handle_mmu_fault' in target/i386/tcg/sysemu/excp_helper.c seems to be the culprit.
In real mode, it fills the TLB with the raw unmasked address. However, for the
protected mode, the `mmu_translate' function does the correct A20 masking.

The fix then should be to just apply the A20 mask in the first branch of the if
statement.

Signed-off-by: Stephen Michael Jothen <sjothen@gmail.com>
---
 target/i386/tcg/sysemu/excp_helper.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/target/i386/tcg/sysemu/excp_helper.c b/target/i386/tcg/sysemu/excp_helper.c
index e1b6d88683..48feba7e75 100644
--- a/target/i386/tcg/sysemu/excp_helper.c
+++ b/target/i386/tcg/sysemu/excp_helper.c
@@ -359,6 +359,7 @@ static int handle_mmu_fault(CPUState *cs, vaddr addr, int size,
     CPUX86State *env = &cpu->env;
     int error_code = PG_ERROR_OK;
     int pg_mode, prot, page_size;
+    int32_t a20_mask;
     hwaddr paddr;
     hwaddr vaddr;
 
@@ -368,7 +369,8 @@ static int handle_mmu_fault(CPUState *cs, vaddr addr, int size,
 #endif
 
     if (!(env->cr[0] & CR0_PG_MASK)) {
-        paddr = addr;
+        a20_mask = x86_get_a20_mask(env);
+        paddr = addr & a20_mask;
 #ifdef TARGET_X86_64
         if (!(env->hflags & HF_LMA_MASK)) {
             /* Without long mode we can only address 32bits in real mode */
-- 
2.30.1 (Apple Git-130)

Re: [PATCH] target/i386/tcg: Fix masking of real-mode addresses with A20 bit

[Paolo Bonzini]

Queued, thanks.

Paolo