linux-kernel - Re: [PATCH v5 7/8] KVM: VMX: Update PID-pointer table entry when APIC ID is changed

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Date:   Fri, 14 Jan 2022 10:17:34 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Chao Gao <chao.gao@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Zeng Guang <guang.zeng@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kim Phillips <kim.phillips@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hu, Robert" <robert.hu@...el.com>
Subject: Re: [PATCH v5 7/8] KVM: VMX: Update PID-pointer table entry when
 APIC ID is changed

On Fri, 2022-01-14 at 10:58 +0800, Chao Gao wrote:
> On Thu, Jan 13, 2022 at 10:19:21PM +0000, Sean Christopherson wrote:
> > On Tue, Jan 11, 2022, Maxim Levitsky wrote:
> > > Both Intel and AMD's PRM also state that changing APIC ID is implementation
> > > dependent.
> > >  
> > > I vote to forbid changing apic id, at least in the case any APIC acceleration
> > > is used, be that APICv or AVIC.
> > 
> > That has my vote as well.  For IPIv in particular there's not much concern with
> > backwards compability, i.e. we can tie the behavior to enable_ipiv.
Great!
> 
> Hi Sean and Levitsky,
> 
> Let's align on the implementation.
> 
> To disable changes for xAPIC ID when IPIv/AVIC is enabled:
> 
> 1. introduce a variable (forbid_apicid_change) for this behavior in kvm.ko
> and export it so that kvm-intel, kvm-amd can set it when IPIv/AVIC is
> enabled. To reduce complexity, this variable is a module level setting.
> 
> 2. when guest attempts to change xAPIC ID but it is forbidden, KVM prints
> a warning on host and injects a #GP to guest.
> 
> 3. remove AVIC code that deals with changes to xAPIC ID.
> 

I have a patch for both, I attached them.
I haven't tested either of these patches that much other than a smoke test,
but I did test all of the guests I  have and none broke in regard to boot.

I will send those patches as part of larger patch series that implements
nesting for AVIC. I hope to do this next week.

Best regards,
	Maxim Levitsky

View attachment "0001-KVM-x86-lapic-don-t-allow-to-change-APIC-ID-when-api.patch" of type "text/x-patch" (1241 bytes)

View attachment "0002-KVM-x86-AVIC-remove-broken-code-that-updated-APIC-ID.patch" of type "text/x-patch" (2144 bytes)