Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Igor Mammedov <imammedo@redhat.com> writes:

> On Mon, 03 Jan 2022 13:56:53 +0100
> Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
>> Igor Mammedov <imammedo@redhat.com> writes:
>> 
>> > On Mon, 03 Jan 2022 09:04:29 +0100
>> > Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>> >  
>> >> Paolo Bonzini <pbonzini@redhat.com> writes:
>> >>   
>> >> > On 12/27/21 18:32, Igor Mammedov wrote:    
>> >> >>> Tweaked and queued nevertheless, thanks.    
>> >> >> it seems this patch breaks VCPU hotplug, in scenario:
>> >> >> 
>> >> >>    1. hotunplug existing VCPU (QEMU stores VCPU file descriptor in parked cpus list)
>> >> >>    2. hotplug it again (unsuspecting QEMU reuses stored file descriptor when recreating VCPU)
>> >> >> 
>> >> >> RHBZ:https://bugzilla.redhat.com/show_bug.cgi?id=2028337#c11
>> >> >>     
>> >> >
>> >> > The fix here would be (in QEMU) to not call KVM_SET_CPUID2 again. 
>> >> > However, we need to work around it in KVM, and allow KVM_SET_CPUID2 if 
>> >> > the data passed to the ioctl is the same that was set before.    
>> >> 
>> >> Are we sure the data is going to be *exactly* the same? In particular,
>> >> when using vCPU fds from the parked list, do we keep the same
>> >> APIC/x2APIC id when hotplugging? Or can we actually hotplug with a
>> >> different id?  
>> >
>> > If I recall it right, it can be a different ID easily.
>> >  
>> 
>> It's broken then. I'd suggest we revert the patch from KVM and think
>> about the strategy how to proceed.
>
> Can you post a patch, then?
>

Paolo,

would you like to send a last minute revert to Linus to save 5.16 ...

>> Going forward, we really want to ban
>> KVM_SET_CPUID{,2} after KVM_RUN (see the comment which my patch moves).
>> E.g. we can have an 'allowlist' of things which can change (and put
>> *APICids there) and only fail KVM_SET_CPUID{,2} when we see something
>> else changing.
>
> It might work, at least on QEMU side we do not allow mixing up CPU models
> within VM instance (so far). So aside of APICid (and related leafs
> (extended APIC ID/possibly other topo related stuff)) nothing else should
> change ever when a new vCPU is hotplugged.

or should we just focus on this (or similar) solution (for 5.17 and
stable@5.16)?

>
>> In QEMU, we can search the parked CPUs list for an entry
>> with the right *APICid and reuse it only if we manage to find one.
> In QEMU, 'parked cpus' fd list is a generic code shared by all supported
> archs. And I'm reluctant to push something x86 specific there (it's not
> impossible, but it's a crutch to workaround forbidden KVM_SET_CPUID{,2}).
>

You can see it this was: vCPU fd corresponds to the particular CPU being
hot plugged/unplugged, not to any CPU in the guest system. The change
may be generic enough then (but it's not going to save existing QEMUs of
course).

-- 
Vitaly