linux-kernel - Re: [PATCH v2] KVM: selftest: Enhance handling WRMSR ICR register in x2APIC mode

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Date:   Fri, 24 Jun 2022 12:28:38 +0800
From:   Zeng Guang <guang.zeng@...el.com>
To:     "Gao, Chao" <chao.gao@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: selftest: Enhance handling WRMSR ICR register in
 x2APIC mode


On 6/23/2022 6:33 PM, Gao, Chao wrote:
> On Thu, Jun 23, 2022 at 05:45:11PM +0800, Zeng Guang wrote:
>
>> +		ASSERT_EQ(icr & ~APIC_ICR_BUSY, val & ~APIC_ICR_BUSY);
> Probably add a comment for it would be better. E.g.,
>
> APIC_ICR_BUSY is removed and not used when CPU is in x2APIC mode.
> It is undefined whether write 1 to this bit will be preserved. So,
> even KVM keeps this bit cleared in some cases even in x2apic mode,
> no guarantee that hardware (specifically, CPU ucode when Intel IPI
> virtualization enabled) will clear the bit. So, skip checking this
> bit.
Hardware won't touch APIC_ICR_BUSY in x2apic mode. It totally depends on 
KVM to
clear it or not if set for test purpose. While in Intel IPI 
virtualization case,
KVM doesn't take care of this bit in vICR writes. So how about the 
comments as
below:

APIC_ICR_BUSY is removed and not used when CPU is in x2APIC mode.
KVM doesn't guarantee to clear this bit in some cases e.g. When
Intel IPI virtualization enabled, if it's set for test purpose.
So, skip checking this bit.

Thanks.
Zeng Guang