lkml.org 
[lkml]   [2022]   [Jan]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 0/5] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug
On Thu, 13 Jan 2022 20:00:08 +0000
Sean Christopherson <seanjc@google.com> wrote:

> On Thu, Jan 13, 2022, Vitaly Kuznetsov wrote:
> > Recently, KVM made it illegal to change CPUID after KVM_RUN but
> > unfortunately this change is not fully compatible with existing VMMs.
> > In particular, QEMU reuses vCPU fds for CPU hotplug after unplug and it
> > calls KVM_SET_CPUID2. Relax the requirement by implementing an allowlist
> > of entries which are allowed to change.
>
> Honestly, I'd prefer we give up and just revert feb627e8d6f6 ("KVM: x86: Forbid
> KVM_SET_CPUID{,2} after KVM_RUN"). Attempting to retroactively restrict the
> existing ioctls is becoming a mess, and I'm more than a bit concerned that this
> will be a maintenance nightmare in the future, without all that much benefit to
> anyone.

in 63f5a1909f9 ("KVM: x86: Alert userspace that KVM_SET_CPUID{,2} after KVM_RUN is broken")
you mention heterogeneous configuration, and that implies that
a userspace (not upstream qemu today) might attempt to change CPUID
and that would be wrong. Do we still care about that?


> I also don't love that the set of volatile entries is nothing more than "this is
> what QEMU needs today". There's no architectural justification, and the few cases
> that do architecturally allow CPUID bits to change are disallowed. E.g. OSXSAVE,
> MONITOR/MWAIT, CPUID.0x12.EAX.SGX1 are all _architecturally_ defined scenarios
> where CPUID can change, yet none of those appear in this list. Some of those are
> explicitly handled by KVM (runtime CPUID updates), but why should it be illegal
> for userspace to intercept writes to MISC_ENABLE and do its own CPUID emulation?
>

\
 
 \ /
  Last update: 2022-01-14 10:09    [W:0.140 / U:1.456 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site