[xen master] x86/hvm: Fix boot on systems where HVM isn't available

[patchbot@xxxxxxx]

commit e9b32164142e40a2585543a46530066b58a76f3f
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Feb 4 17:01:41 2022 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Feb 7 17:41:24 2022 +0000

    x86/hvm: Fix boot on systems where HVM isn't available
    
    c/s 27a63cdac388 ("x86/HVM: convert remaining hvm_funcs hook invocations to
    alt-call") went too far with dropping NULL function pointer checks.
    
    smp_callin() and S3 resume call hvm_cpu_up() unconditionally.  When the
    platform doesn't support HVM, hvm_enable() exits without filling in 
hvm_funcs,
    after which the altcall pass nukes the (now unconditional) indirect call,
    causing:
    
      (XEN) ----[ Xen-4.17.0-10.18-d  x86_64  debug=y  Not tainted ]----
      (XEN) CPU:    1
      (XEN) RIP:    e008:[<ffff82d04034bef5>] start_secondary+0x393/0x3b7
      (XEN) RFLAGS: 0000000000010086   CONTEXT: hypervisor
      ...
      (XEN) Xen code around <ffff82d04034bef5> (start_secondary+0x393/0x3b7):
      (XEN)  ff ff 8b 05 1b 84 17 00 <0f> 0b 0f ff ff 90 89 c3 85 c0 0f 84 db 
fe ff ff
      ...
      (XEN) Xen call trace:
      (XEN)    [<ffff82d04034bef5>] R start_secondary+0x393/0x3b7
      (XEN)    [<ffff82d0402000e2>] F __high_start+0x42/0x60
    
    To make matters worse, several paths including __stop_this_cpu() call
    hvm_cpu_down() unconditionally too, so what happen next is:
    
      (XEN) ----[ Xen-4.17.0-10.18-d  x86_64  debug=y  Not tainted ]----
      (XEN) CPU:    0
      (XEN) RIP:    e008:[<ffff82d04034ab02>] __stop_this_cpu+0x12/0x3c
      (XEN) RFLAGS: 0000000000010046   CONTEXT: hypervisor
      ...
      (XEN) Xen code around <ffff82d04034ab02> (__stop_this_cpu+0x12/0x3c):
      (XEN)  48 89 e5 e8 8a 1d fd ff <0f> 0b 0f ff ff 90 0f 06 db e3 48 89 e0 
48 0d ff
      ...
      (XEN) Xen call trace:
      (XEN)    [<ffff82d04034ab02>] R __stop_this_cpu+0x12/0x3c
      (XEN)    [<ffff82d04034ac15>] F smp_send_stop+0xdd/0xf8
      (XEN)    [<ffff82d04034a229>] F machine_restart+0xa2/0x298
      (XEN)    [<ffff82d04034a42a>] F 
arch/x86/shutdown.c#__machine_restart+0xb/0x11
      (XEN)    [<ffff82d04022fd15>] F smp_call_function_interrupt+0xbf/0xea
      (XEN)    [<ffff82d04034acc6>] F call_function_interrupt+0x35/0x37
      (XEN)    [<ffff82d040331a70>] F do_IRQ+0xa3/0x6b5
      (XEN)    [<ffff82d04039482a>] F common_interrupt+0x10a/0x120
      (XEN)    [<ffff82d04031f649>] F __udelay+0x3a/0x51
      (XEN)    [<ffff82d04034d5fb>] F __cpu_up+0x48f/0x734
      (XEN)    [<ffff82d040203c2b>] F cpu_up+0x7d/0xde
      (XEN)    [<ffff82d0404543d3>] F __start_xen+0x200b/0x2618
      (XEN)    [<ffff82d0402000ef>] F __high_start+0x4f/0x60
    
    which recurses until hitting a stack overflow.  The #DF handler, which 
resets
    its stack on each invocation, loops indefinitely.
    
    Reinstate the NULL function pointer checks for hvm_cpu_{up,down}(), along 
with
    comments explaining how the helpers are used.
    
    Fixes: 27a63cdac388 ("x86/HVM: convert remaining hvm_funcs hook invocations 
to alt-call")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/include/asm/hvm/hvm.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/include/asm/hvm/hvm.h 
b/xen/arch/x86/include/asm/hvm/hvm.h
index a441cbc221..b44bbdeb21 100644
--- a/xen/arch/x86/include/asm/hvm/hvm.h
+++ b/xen/arch/x86/include/asm/hvm/hvm.h
@@ -551,14 +551,20 @@ static inline void hvm_invlpg(struct vcpu *v, unsigned 
long linear)
                        (1U << TRAP_alignment_check) | \
                        (1U << TRAP_machine_check))
 
+/* Called in boot/resume paths.  Must cope with no HVM support. */
 static inline int hvm_cpu_up(void)
 {
-    return alternative_call(hvm_funcs.cpu_up);
+    if ( hvm_funcs.cpu_up )
+        return alternative_call(hvm_funcs.cpu_up);
+
+    return 0;
 }
 
+/* Called in shutdown paths.  Must cope with no HVM support. */
 static inline void hvm_cpu_down(void)
 {
-    alternative_vcall(hvm_funcs.cpu_down);
+    if ( hvm_funcs.cpu_down )
+        alternative_vcall(hvm_funcs.cpu_down);
 }
 
 static inline unsigned int hvm_get_insn_bytes(struct vcpu *v, uint8_t *buf)
--
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