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

[Sean Christopherson]

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.