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Re: [Xen-devel] [PATCH v2] x86/xen: Fix 32-bit PV guests's usage of kernel_stack

On 04/10/2014 02:20 PM, David Vrabel wrote:
On 10/04/14 17:17, Boris Ostrovsky wrote:
Commit 198d208df4371734ac4728f69cb585c284d20a15 ("x86: Keep thread_info
on thread stack in x86_32") made 32-bit kernels use kernel_stack to point to
thread_info. That change missed a couple of updates needed by Xen's
32-bit PV guests:

1. kernel_stack needs to be initialized for secondary CPUs
2. GET_THREAD_INFO() now uses %fs register which may not be the kernel's
version when executing xen_iret().

With respect to the second issue, we don't need GET_THREAD_INFO()
anymore: we used it as an intermediate step to get to per_cpu xen_vcpu and avoid
referencing %fs. Now that we are going to use %fs anyway we may as well go
directly to xen_vcpu.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
  arch/x86/xen/smp.c        |    3 ++-
  arch/x86/xen/xen-asm_32.S |   25 +++++++++++++++++--------
  2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index a18eadd..7005974 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -441,10 +441,11 @@ static int xen_cpu_up(unsigned int cpu, struct 
task_struct *idle)
        clear_tsk_thread_flag(idle, TIF_FORK);
        per_cpu(kernel_stack, cpu) =
                (unsigned long)task_stack_page(idle) -
diff --git a/arch/x86/xen/xen-asm_32.S b/arch/x86/xen/xen-asm_32.S
index 33ca6e4..fd92a64 100644
--- a/arch/x86/xen/xen-asm_32.S
+++ b/arch/x86/xen/xen-asm_32.S
@@ -75,6 +75,17 @@ ENDPROC(xen_sysexit)
   * stack state in whatever form its in, we keep things simple by only
   * using a single register which is pushed/popped on the stack.
+.macro POP_FS
+       popw %fs
+.pushsection .fixup, "ax"
+2:     movw $0, (%esp)
+       jmp 1b
+       _ASM_EXTABLE(1b,2b)
        /* test eflags for special cases */
        testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp)
@@ -83,15 +94,13 @@ ENTRY(xen_iret)
        push %eax
        ESP_OFFSET=4    # bytes pushed onto stack
- /*
-        * Store vcpu_info pointer for easy access.  Do it this way to
-        * avoid having to reload %fs
-        */
+       /* Store vcpu_info pointer for easy access */
  #ifdef CONFIG_SMP
-       GET_THREAD_INFO(%eax)
-       movl %ss:TI_cpu(%eax), %eax
-       movl %ss:__per_cpu_offset(,%eax,4), %eax
-       mov %ss:xen_vcpu(%eax), %eax
+       pushw %fs
+       movl $(__KERNEL_PERCPU), %eax
+       movl %eax, %fs
+       movl %fs:xen_vcpu, %eax
How can this get the correct per-cpu xen_vcpu pointer if it doesn't ever
get the current cpu number?  Doesn't this always get VCPU#0's xen_vcpu?

%fs is pointing to per-cpu segment so %fs:xen_vcpu should be different on each (V)CPU.


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