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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.02.2022 10:47, Roger Pau Monné wrote:
> On Fri, Feb 04, 2022 at 05:34:05PM +0000, Andrew Cooper wrote:
>> c/s 27a63cdac388 ("x86/HVM: convert remaining hvm_funcs hook invocations to
>> alt-call") went too far with dropping NULL function pointer checks.

Oh, I'm sorry, I should have noticed this.

>> smp_callin() calls 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, __stop_this_cpu() calls 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}().
>>
>> 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>

>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Wei Liu <wl@xxxxxxx>
>>
>> RFC.  Not tested yet on the imacted hardware.  It's a Xeon PHI with another
>> werid thing in need of debugging.  First boot is fine, while second
>> boot (loading microcode this time) has a problem with vmx.

Sounds not unfamiliar: My meanwhile oldish Romley needs to be cold-
booted for VMX to actually be usable (not locked) on APs.

>> I wonder if we want to modify the callers to check for HVM being enabled,
>> rather than leaving the NULL pointer checks in a position where they're 
>> liable
>> to be reaped again.
> 
> What about adding a couple of comments to hvm_cpu_{up,down} to note
> they are called unconditionally regardless of whether HVM is present
> or not?

I second this as the perhaps better alternative: The S3 path is
similarly affected (and you may want to mention this in the
description), so this would mean up to 5 conditionals (at the
source level) instead of the just two you get away with here.

Jan