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

[Chao Gao]

On Mon, Jan 17, 2022 at 09:35:04PM +0800, Chao Gao wrote:
>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()));

Sorry. It should be:
	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.