lkml.org 
[lkml]   [2022]   [Jan]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN
Date
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. 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. 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.

--
Vitaly

\
 
 \ /
  Last update: 2022-01-03 13:57    [W:0.180 / U:0.112 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site