Re: [PATCH 5/6] KVM: x86: Remove WARN_ON in kvm_arch_check_processor_compat

[Chao Gao]

On Wed, Jan 12, 2022 at 05:35:12PM +0000, Sean Christopherson wrote:
>On Wed, Jan 12, 2022, Chao Gao wrote:
>> On Tue, Jan 11, 2022 at 07:48:39PM +0000, Sean Christopherson wrote:
>> >On Tue, Jan 11, 2022, Tian, Kevin wrote:
>> >> > From: Sean Christopherson <seanjc@xxxxxxxxxx>
>> >> > Sent: Tuesday, January 11, 2022 7:00 AM
>> >> > 
>> >> > On Mon, Dec 27, 2021, Chao Gao wrote:
>> >> > > kvm_arch_check_processor_compat() needn't be called with interrupt
>> >> > > disabled, as it only reads some CRs/MSRs which won't be clobbered
>> >> > > by interrupt handlers or softirq.
>> >> > >
>> >> > > What really needed is disabling preemption. No additional check is
>> >> > > added because if CONFIG_DEBUG_PREEMPT is enabled, smp_processor_id()
>> >> > > (right above the WARN_ON()) can help to detect any violation.
>> >> > 
>> >> > Hrm, IIRC, the assertion that IRQs are disabled was more about detecting
>> >> > improper usage with respect to KVM doing hardware enabling than it was
>> >> > about ensuring the current task isn't migrated.  E.g. as exhibited by patch
>> >> > 06, extra protections (disabling of hotplug in that case) are needed if
>> >> > this helper is called outside of the core KVM hardware enabling flow since
>> >> > hardware_enable_all() does its thing via SMP function call.
>> >> 
>> >> Looks the WARN_ON() was added by you. 😊
>> >
>> >Yeah, past me owes current me a beer.
>> >
>> >> commit f1cdecf5807b1a91829a2dc4f254bfe6bafd4776
>> >> Author: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>> >> Date:   Tue Dec 10 14:44:14 2019 -0800
>> >> 
>> >>     KVM: x86: Ensure all logical CPUs have consistent reserved cr4 bits
>> >> 
>> >>     Check the current CPU's reserved cr4 bits against the mask calculated
>> >>     for the boot CPU to ensure consistent behavior across all CPUs.
>> >> 
>> >>     Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>> >>     Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> >> 
>> >> But it's unclear to me how this WARN_ON() is related to what the commit
>> >> msg tries to explain.
>> >
>> >Ya, the changelog and lack of a comment is awful.
>> >
>> >> When I read this code it's more like a sanity check on the assumption that it
>> >> is currently called in SMP function call which runs the said function with
>> >> interrupt disabled.
>> >
>> >Yes, and as above, that assertion was more about the helper not really being safe
>> >for general usage as opposed to wanting to detect use from preemptible context.
>> >If we end up keeping the WARN_ON, I'll happily write a comment explaining the
>> >point of the assertion.
>> 
>> OK. I will do following changes to keep the WARN_ON():
>> 1. drop this patch
>> 2. disable interrupt before the call site in patch 6.
>
>No, we shouldn't sully other code just to keep this WARN.  Again, the point of
>the WARN is/was to highlight that any use outside of the hardware enabling path
>is suspect.  That's why I asked if there was a way this code could identify that
>the CPU in question is being hotplugged, i.e. to convey that the helper is safe
>to use only during hardware enabling _or_ hotplug.  If that's not feasible,
>replacing the WARN with a scary comment is better than disabling IRQs.

OK. How about:

	/*
	 * Compatibility checks are done when loading KVM or in KVM's CPU
	 * hotplug callback. It ensures all online CPUs are compatible before
	 * running any vCPUs. For other cases, compatibility checks are
	 * unnecessary or even problematic. Try to detect improper usages here.
	 */
	WARN_ON(!irqs_disabled() && !cpu_active(smp_processor_id()));

a CPU is active when it reaches the CPUHP_AP_ACTIVE state (the last state before
CPUHP_ONLINE). So, if a cpu isn't active, it probably is being hotplugged. One
false positive is the CPU is dying, which I guess is fine.

And to help justify this change, I will merge it into patch 6.