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

From: Chao Gao <chao.gao@intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/6] KVM: x86: Remove WARN_ON in kvm_arch_check_processor_compat
Date: Wed, 12 Jan 2022 19:00:01 +0800	[thread overview]
Message-ID: <20220112110000.GA10249@gao-cwp> (raw)
In-Reply-To: <Yd3fFxg3IjWPUIqH@google.com>

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@google.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@intel.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@intel.com>
>>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.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.