Re: [PATCH RFC 0/2] KVM: nVMX: fix apicv disablement for L1

[Sean Christopherson]

On Thu, Feb 20, 2020 at 06:22:03PM +0100, Vitaly Kuznetsov wrote:
> It was found that fine-grained VMX feature enablement in QEMU is broken
> when combined with SynIC:
> 
>     qemu-system-x86_64 -machine q35,accel=kvm -cpu host,hv_vpindex,hv_synic -smp 2 -m 16384 -vnc :0
>     qemu-system-x86_64: error: failed to set MSR 0x48d to 0xff00000016
>     qemu-system-x86_64: <...>: kvm_buf_set_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.
>     Aborted
> 
> QEMU thread: https://lists.gnu.org/archive/html/qemu-devel/2020-02/msg04838.html
> 
> Turns out, this is a KVM issue: when SynIC is enabled, PIN_BASED_POSTED_INTR
> gets filtered out from VMX MSRs for all newly created (but not existent!)
> vCPUS. Patch1 addresses this. Also, apicv disablement for L1 doesn't seem
> to disable it for L2 (at least on CPU0) so unless there's a good reason
> to not allow this we need to make it work. PATCH2, suggested by Paolo,
> is supposed to do the job.
> 
> RFC: I looked at the code and ran some tests and nothing suspicious popped
> out, however, I'm still not convinced this is a good idea to have apicv
> enabled for L2 when it's disabled for L1...

Eh, if it works...  IMO, enabling apicv for both L1 and L2 is far more
mindbending.

> Also, we may prefer to merge or re-order these two patches.

It does seem like patch 2 should be applied first.  

> Vitaly Kuznetsov (2):
>   KVM: nVMX: clear PIN_BASED_POSTED_INTR from nested pinbased_ctls only
>     when apicv is globally disabled
>   KVM: nVMX: handle nested posted interrupts when apicv is disabled for
>     L1
> 
>  arch/x86/include/asm/kvm_host.h |  2 +-
>  arch/x86/kvm/lapic.c            |  5 +----
>  arch/x86/kvm/svm.c              |  7 ++++++-
>  arch/x86/kvm/vmx/capabilities.h |  1 +
>  arch/x86/kvm/vmx/nested.c       |  5 ++---
>  arch/x86/kvm/vmx/nested.h       |  3 +--
>  arch/x86/kvm/vmx/vmx.c          | 23 +++++++++++++----------
>  7 files changed, 25 insertions(+), 21 deletions(-)
> 
> -- 
> 2.24.1
>