ACK/Cmnt: [SRU][Focal][PATCH 1/1] KVM: x86: Set KVM_REQ_EVENT if run is canceled with req_immediate_exit set

[Stefan Bader]

On 19.01.21 00:02, Matthew Ruffell wrote:
> From: Sean Christopherson <sean.j.christopherson at intel.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1911848
> 
> Re-request KVM_REQ_EVENT if vcpu_enter_guest() bails after processing
> pending requests and an immediate exit was requested.  This fixes a bug
> where a pending event, e.g. VMX preemption timer, is delayed and/or lost
> if the exit was deferred due to something other than a higher priority
> _injected_ event, e.g. due to a pending nested VM-Enter.  This bug only
> affects the !injected case as kvm_x86_ops.cancel_injection() sets
> KVM_REQ_EVENT to redo the injection, but that's purely serendipitous
> behavior with respect to the deferred event.
> 
> Note, emulated preemption timer isn't the only event that can be
> affected, it simply happens to be the only event where not re-requesting
> KVM_REQ_EVENT is blatantly visible to the guest.
> 
> Fixes: f4124500c2c13 ("KVM: nVMX: Fully emulate preemption timer")
> Signed-off-by: Sean Christopherson <sean.j.christopherson at intel.com>
> Message-Id: <20200423022550.15113-4-sean.j.christopherson at intel.com>
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> (backported from commit 8081ad06b68a728e676d3b08e9ab70ce4039747b)
> [mruffell: minor context adjustment]
> Signed-off-by: Matthew Ruffell <matthew.ruffell at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>
> ---

The regression potential is probably covering all bases but also sounding rather
scary that way. I would say practically the change adds an exit case from
immediate re-schedule (iiuc). So regression potential would be stuck guests with
no cpu activity instead of 100% as before. Yeah, not sure that sounds better.
But at least a little more specific.

-Stefan

>  arch/x86/kvm/x86.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e519e0af028a..dddbeb3ee2c6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8345,6 +8345,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	return r;
>  
>  cancel_injection:
> +	if (req_immediate_exit)
> +		kvm_make_request(KVM_REQ_EVENT, vcpu);
>  	kvm_x86_ops->cancel_injection(vcpu);
>  	if (unlikely(vcpu->arch.apic_attention))
>  		kvm_lapic_sync_from_vapic(vcpu);
> 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: OpenPGP digital signature
URL: <https://lists.ubuntu.com/archives/kernel-team/attachments/20210121/fece2b64/attachment.sig>