linux-kernel - Re: [PATCH v6 6/9] KVM: x86: lapic: don't allow to change APIC ID unconditionally

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Message-ID: <57e91d8f75d6e39432a2fef5d899e3238154863f.camel@redhat.com>
Date:   Fri, 25 Feb 2022 18:12:41 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Zeng Guang <guang.zeng@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <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>,
        Kai Huang <kai.huang@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Robert Hu <robert.hu@...el.com>, Gao Chao <chao.gao@...el.com>
Subject: Re: [PATCH v6 6/9] KVM: x86: lapic: don't allow to change APIC ID
 unconditionally

On Fri, 2022-02-25 at 15:42 +0000, David Woodhouse wrote:
> On Fri, 2022-02-25 at 17:11 +0200, Maxim Levitsky wrote:
> > On Fri, 2022-02-25 at 14:56 +0000, David Woodhouse wrote:
> > > On Fri, 2022-02-25 at 16:46 +0200, Maxim Levitsky wrote:
> > > > Assuming that this is approved and accepted upstream,
> > > > that is even better that my proposal of doing this
> > > > when APICv is enabled.
> > > > 
> > > > Since now apic id is always read only, now we should not 
> > > > forget to clean up some parts of kvm like kvm_recalculate_apic_map,
> > > > which are not needed anymore
> > > 
> > > Can we also now optimise kvm_get_vcpu_by_id() so it doesn't have to do
> > > a linear search over all the vCPUs when there isn't a 1:1
> > > correspondence with the vCPU index?
> > 
> > I don't think so since vcpu id can still be set by userspace to anything,
> > and this is even used to encode topology in it.
> 
> Yes, but it can only be set at vCPU creation time and it has to be
> unique.
> 
> > However a hash table can still be used there to speed it up regardless of
> > read-only apic id IMHO.
> > 
> > Or, even better than a hash table, I see that KVM already 
> > limits vcpu_id to KVM_MAX_VCPUS * 4 with a comment that only two extra
> > bits of topology are used:
> 
> We already have the kvm_apic_map which provides a fast lookup. The key
> point here is that the APIC ID can't *change* from vcpu->vcpu_id any
> more, so we can actually use the kvm_apic_map for kvm_get_vcpu_by_id()
> now, can't we?
> 
Right! I wrote my response partially when I still assumed that vcpu_id
can be any 32 bit number (thus hash table), 
and later checked that it is capped by KVM_MAX_VCPUS * 4 which isn't a big number, 
plus as I now see in the kvm_recalculate_apic_map
the map is dynamically allocated up to the max apic id.

(technically speaking an array is a hash table).

Now the map would only be needed to be rebuit few times when new vCPUs are added,
and can be used to locate vcpu by its apic id.

I so hope that this patch is accepted so all of this could be done.

Best regards,
	Maxim Levitsky