linux-kernel - Re: [External] Re: [PATCH v10 6/8] x86/smpboot: Support parallel startup of secondary CPUs

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Date:   Fri, 24 Feb 2023 19:16:52 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Yuan Yao <yuan.yao@...ux.intel.com>
Cc:     tglx@...utronix.de, kim.phillips@....com, piotrgorski@...hyos.org,
        oleksandr@...alenko.name, arjan@...ux.intel.com, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        x86@...nel.org, pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com
Subject: Re: [External] Re: [PATCH v10 6/8] x86/smpboot: Support parallel
 startup of secondary CPUs



On 24/02/2023 15:46, David Woodhouse wrote:
> On Fri, 2023-02-24 at 13:59 +0800, Yuan Yao wrote:
>>
>>> +        * Make sure only one CPU fiddles with the realmode stack
>>> +        */
>>> +.Llock_rm:
>>> +       btl     $0, tr_lock
>>> +       jnc     2f
>>> +       pause
>>> +       jmp     .Llock_rm
>>> +2:
>>> +       lock
>>> +       btsl    $0, tr_lock
>>> +       jc      .Llock_rm
>>> +
>>
>> Looks these changes should be applied to trampoline_start64()
>> yet, which is used for boot up APs when apic->wakeup_secondary_cpu_64
>> is available, e.g when ACPI_MADT_TYPE_MULTIPROC_WAKEUP is available.
>>
>> One case I know is the INTEL TD guest, which using the MADT wakeup
>> for AP wake up now.
> 
> Yeah.
> 
> I think we could probably pull that all out into a separate asm
> "load_realmode_esp" function which takes the lock and actually does the
> 'movl $rm_stack_end, %esp'.
> 
> Then we call it from all the places which currently use $rm_stack_end,
> including sev_es_trampoline_start (even though we currently disable
> parallel startup there because CPUID doesn't work that early).
> 
> Oh... except of course it can't be a function because we haven't got a
> stack, have we? It's a macro.
> 
> Usama, are you happy using .macro in gas?

Yes, makes sense. I guess something like below should be ok?
It makes sense to add it for sev as well.

diff --git a/arch/x86/realmode/rm/trampoline_64.S 
b/arch/x86/realmode/rm/trampoline_64.S
index e38d61d6562e..3a724d8d85b9 100644
--- a/arch/x86/realmode/rm/trampoline_64.S
+++ b/arch/x86/realmode/rm/trampoline_64.S
@@ -38,6 +38,25 @@
         .code16

         .balign PAGE_SIZE
+
+.macro LOAD_REALMODE_ESP
+       /*
+        * Make sure only one CPU fiddles with the realmode stack
+        */
+.Llock_rm\@:
+       btl     $0, tr_lock
+       jnc     2f
+       pause
+       jmp     .Llock_rm\@
+2:
+       lock
+       btsl    $0, tr_lock
+       jc      .Llock_rm\@
+
+       # Setup stack
+       movl    $rm_stack_end, %esp
+.endm
+
  SYM_CODE_START(trampoline_start)
         cli                     # We should be safe anyway
         wbinvd
@@ -49,8 +68,7 @@ SYM_CODE_START(trampoline_start)
         mov     %ax, %es
         mov     %ax, %ss

-       # Setup stack
-       movl    $rm_stack_end, %esp
+       LOAD_REALMODE_ESP

         call    verify_cpu              # Verify the cpu supports long mode
         testl   %eax, %eax              # Check for return code
@@ -93,8 +111,7 @@ SYM_CODE_START(sev_es_trampoline_start)
         mov     %ax, %es
         mov     %ax, %ss

-       # Setup stack
-       movl    $rm_stack_end, %esp
+       LOAD_REALMODE_ESP

         jmp     .Lswitch_to_protected
  SYM_CODE_END(sev_es_trampoline_start)
@@ -177,7 +194,7 @@ SYM_CODE_START(pa_trampoline_compat)
          * In compatibility mode.  Prep ESP and DX for startup_32, then 
disable
          * paging and complete the switch to legacy 32-bit mode.
          */
-       movl    $rm_stack_end, %esp
+       LOAD_REALMODE_ESP
         movw    $__KERNEL_DS, %dx

         movl    $(CR0_STATE & ~X86_CR0_PG), %eax