[PATCH] target/i386: fix access to the T bit of the TSS

[PATCH] target/i386: fix access to the T bit of the TSS

[Paolo Bonzini]

The T bit is bit 0 of the 16-bit word at offset 100 of the TSS.  However,
accessing it with a 32-bit word is not really correct, because bytes
102-103 contain the I/O map base address (relative to the base of the
TSS) and bits 1-15 are reserved.  In particular, any task switch to a TSS that
has a nonzero I/O map base address is broken.

This fixes the eventinj and taskswitch tests in kvm-unit-tests.

Cc: qemu-stable@nongnu.org
Fixes: ad441b8b791 ("target/i386: implement TSS trap bit", 2025-05-12)
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/tcg/seg_helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/i386/tcg/seg_helper.c b/target/i386/tcg/seg_helper.c
index 071f3fbd83d..f49fe851cdf 100644
--- a/target/i386/tcg/seg_helper.c
+++ b/target/i386/tcg/seg_helper.c
@@ -456,7 +456,7 @@ static void switch_tss_ra(CPUX86State *env, int tss_selector,
             new_segs[i] = access_ldw(&new, tss_base + (0x48 + i * 4));
         }
         new_ldt = access_ldw(&new, tss_base + 0x60);
-        new_trap = access_ldl(&new, tss_base + 0x64);
+        new_trap = access_ldw(&new, tss_base + 0x64) & 1;
     } else {
         /* 16 bit */
         new_cr3 = 0;
-- 
2.51.0

Re: [PATCH] target/i386: fix access to the T bit of the TSS

[Thomas Huth]

On 13/10/2025 18.11, Paolo Bonzini wrote:
> The T bit is bit 0 of the 16-bit word at offset 100 of the TSS.  However,
> accessing it with a 32-bit word is not really correct, because bytes
> 102-103 contain the I/O map base address (relative to the base of the
> TSS) and bits 1-15 are reserved.  In particular, any task switch to a TSS that
> has a nonzero I/O map base address is broken.
> 
> This fixes the eventinj and taskswitch tests in kvm-unit-tests.
> 
> Cc: qemu-stable@nongnu.org
> Fixes: ad441b8b791 ("target/i386: implement TSS trap bit", 2025-05-12)
> Reported-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Closes: https://gitlab.com/qemu-project/qemu/-/issues/3101
Tested-by: Thomas Huth <thuth@redhat.com>