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

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Message-ID: <b9638fe3383d7b36846255e1d05afa9c1bfc7a0f.camel@redhat.com>
Date:   Fri, 25 Feb 2022 17:11:11 +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 14:56 +0000, David Woodhouse wrote:
> On Fri, 2022-02-25 at 16:46 +0200, Maxim Levitsky wrote:
> > On Fri, 2022-02-25 at 16:22 +0800, Zeng Guang wrote:
> > > From: Maxim Levitsky <
> > > mlevitsk@...hat.com
> > > 
> > > No normal guest has any reason to change physical APIC IDs, and
> > > allowing this introduces bugs into APIC acceleration code.
> > > 
> > > And Intel recent hardware just ignores writes to APIC_ID in
> > > xAPIC mode. More background can be found at:
> > > https://lore.kernel.org/lkml/Yfw5ddGNOnDqxMLs@google.com/
> > > 
> > > 
> > > Looks there is no much value to support writable xAPIC ID in
> > > guest except supporting some old and crazy use cases which
> > > probably would fail on real hardware. So, make xAPIC ID
> > > read-only for KVM guests.
> > > 
> > > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > > Signed-off-by: Zeng Guang <guang.zeng@...el.com>
> > 
> > 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.


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:

"In the worst case, we'll need less than one extra bit for the
 * Core ID, and less than one extra bit for the Package (Die) ID,
 * so ratio of 4 should be enough"

Thus, we could in theory standardize location of those bits in apic_id 
(even with a new KVM extension and do linear search for legacy userspace), 
and then just mask/shift the topology bits.

The kvm extension would be defining how many low (or high?) bits of vcpu_id
are topology bits.

Best regards,
	Maxim Levitsky