linux-kernel - Re: [PATCH 5/6] KVM: x86: Remove WARN_ON in kvm_arch_check_processor

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Date:   Wed, 12 Jan 2022 19:00:01 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] KVM: x86: Remove WARN_ON in
 kvm_arch_check_processor_compat

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@...gle.com>
>> > 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@...el.com>
>> 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@...el.com>
>>     Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> 
>> 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.