Re: [PATCH 1/2] x86: Enable x2apic during resume from suspend if used previously

[Mario Limonciello <mario.limonciello@amd.com>]

+Tom

On 10/24/2023 03:36, Ingo Molnar wrote:
> 
> * Mario Limonciello <mario.limonciello@amd.com> wrote:
> 
>> If x2apic was enabled during boot with parallel startup
>> it will be needed during resume from suspend to ram as well.
>>
>> Store whether to enable into the smpboot_control global variable
>> and during startup re-enable it if necessary.
>>
>> Cc: stable@vger.kernel.org # 6.5+
>> Fixes: 0c7ffa32dbd6 ("x86/smpboot/64: Implement arch_cpuhp_init_parallel_bringup() and enable it")
>> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
>> ---
>>   arch/x86/include/asm/smp.h   |  1 +
>>   arch/x86/kernel/acpi/sleep.c | 12 ++++++++----
>>   arch/x86/kernel/head_64.S    | 15 +++++++++++++++
>>   3 files changed, 24 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
>> index c31c633419fe..86584ffaebc3 100644
>> --- a/arch/x86/include/asm/smp.h
>> +++ b/arch/x86/include/asm/smp.h
>> @@ -190,6 +190,7 @@ extern unsigned long apic_mmio_base;
>>   #endif /* !__ASSEMBLY__ */
>>   
>>   /* Control bits for startup_64 */
>> +#define STARTUP_ENABLE_X2APIC	0x40000000
>>   #define STARTUP_READ_APICID	0x80000000
>>   
>>   /* Top 8 bits are reserved for control */
>> diff --git a/arch/x86/kernel/acpi/sleep.c b/arch/x86/kernel/acpi/sleep.c
>> index 6dfecb27b846..29734a1299f6 100644
>> --- a/arch/x86/kernel/acpi/sleep.c
>> +++ b/arch/x86/kernel/acpi/sleep.c
>> @@ -11,6 +11,7 @@
>>   #include <linux/dmi.h>
>>   #include <linux/cpumask.h>
>>   #include <linux/pgtable.h>
>> +#include <asm/apic.h>
>>   #include <asm/segment.h>
>>   #include <asm/desc.h>
>>   #include <asm/cacheflush.h>
>> @@ -129,11 +130,14 @@ int x86_acpi_suspend_lowlevel(void)
>>   	 */
>>   	current->thread.sp = (unsigned long)temp_stack + sizeof(temp_stack);
>>   	/*
>> -	 * Ensure the CPU knows which one it is when it comes back, if
>> -	 * it isn't in parallel mode and expected to work that out for
>> -	 * itself.
>> +	 * Ensure x2apic is re-enabled if necessary and the CPU knows which
>> +	 * one it is when it comes back, if it isn't in parallel mode and
>> +	 * expected to work that out for itself.
>>   	 */
>> -	if (!(smpboot_control & STARTUP_PARALLEL_MASK))
>> +	if (smpboot_control & STARTUP_PARALLEL_MASK) {
>> +		if (x2apic_enabled())
>> +			smpboot_control |= STARTUP_ENABLE_X2APIC;
>> +	} else
>>   		smpboot_control = smp_processor_id();
> 
> Yeah, so instead of adding further kludges to the 'parallel bringup is
> possible' code path, which is arguably a functional feature that shouldn't
> have hardware-management coupled to it, would it be possible to fix
> parallel bringup to AMD-SEV systems, so that this code path isn't a
> quirk-dependent "parallel boot" codepath, but simply the "x86 SMP boot
> codepath", where all SMP x86 systems do a parallel bootup?
> 
> The original commit by Thomas says:
> 
>    0c7ffa32dbd6 ("x86/smpboot/64: Implement arch_cpuhp_init_parallel_bringup() and enable it")
> 
>    | Unfortunately there is no RDMSR GHCB protocol at the moment, so enabling
>    | AMD-SEV guests for parallel startup needs some more thought.
> 
> But that was half a year ago, isn't there RDMSR GHCB access code available now?
> 
> This code would all read a lot more natural if it was the regular x86 SMP
> bootup path - which it is 'almost' today already, modulo quirk.
> 
> Obviously coupling functional features with hardware quirks is fragile, for
> example your patch extending x86 SMP parallel bringup doesn't extend the
> AMD-SEV case, which may or may not matter in practice.
> 
> So, if it's possible, it would be nice to fix AMD-SEV systems as well and
> remove this artificial coupling.

It probably isn't clear since I didn't mention it in the commit message, 
but this is not a system that supports AMD-SEV.  This is a workstation 
that supports x2apic.  I'll clarify that for V2.

I've looped Tom in to comment whether it's possible to improve AMD-SEV 
as well.

> 
> Also, side note #1: curly braces should be balanced.
> 
>>   #endif
>>   	initial_code = (unsigned long)wakeup_long64;
>> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
>> index ea6995920b7a..fcfa79105928 100644
>> --- a/arch/x86/kernel/head_64.S
>> +++ b/arch/x86/kernel/head_64.S
>> @@ -236,10 +236,15 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
>>   	 * used to look up the CPU number.  For booting a single CPU, the
>>   	 * CPU number is encoded in smpboot_control.
>>   	 *
>> +	 * Bit 30	STARTUP_ENABLE_X2APIC (Enable X2APIC mode)
>>   	 * Bit 31	STARTUP_READ_APICID (Read APICID from APIC)
>>   	 * Bit 0-23	CPU# if STARTUP_xx flags are not set
> 
> Side note #2: you mixed up the comment ordering here.
> 
> Thanks,
> 
> 	Ingo

Sure, thanks for the feedback.  I'll adjust the style for v2.