Re: [PATCH 1/2] KVM: VMX: Disable posted interrupts for odd IRQs

[Liran Alon <liran.alon@oracle.com>]

> On 3 Sep 2019, at 17:29, Alexander Graf <graf@amazon.com> wrote:
> 
> We can easily route hardware interrupts directly into VM context when
> they target the "Fixed" or "LowPriority" delivery modes.
> 
> However, on modes such as "SMI" or "Init", we need to go via KVM code
> to actually put the vCPU into a different mode of operation, so we can
> not post the interrupt

I would also mention in commit message that one can see this is also
true in KVM’s vLAPIC code. i.e. __apic_accept_irq() can call
kvm_x86_ops->deliver_posted_interrupt() only in case deliver-mode is
either “Fixed” or “LowPriority”. 

> 
> Add code in the VMX PI logic to explicitly refuse to establish posted
> mappings for advanced IRQ deliver modes.
> 
> This fixes a bug I have with code which configures real hardware to
> inject virtual SMIs into my guest.
> 
> Signed-off-by: Alexander Graf <graf@amazon.com>

With some small improvements I written inline below:
Reviewed-by: Liran Alon <liran.alon@oracle.com>

> ---
> arch/x86/kvm/vmx/vmx.c | 22 ++++++++++++++++++++++
> 1 file changed, 22 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 570a233e272b..d16c4ae8f685 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7401,6 +7401,28 @@ static int vmx_update_pi_irte(struct kvm *kvm, unsigned int host_irq,
> 			continue;
> 		}
> 
> +		switch (irq.delivery_mode) {
> +		case dest_Fixed:
> +		case dest_LowestPrio:
> +			break;
> +		default:
> +			/*
> +			 * For non-trivial interrupt events, we need to go
> +			 * through the full KVM IRQ code, so refuse to take
> +			 * any direct PI assignments here.
> +			 */
> +
> +			ret = irq_set_vcpu_affinity(host_irq, NULL);
> +			if (ret < 0) {
> +				printk(KERN_INFO
> +				   "failed to back to remapped mode, irq: %u\n",
> +				   host_irq);
> +				goto out;

I recommend we will chose to print here a string that is different than the !kvm_intr_is_single_vcpu()
case to make it easier to diagnose which case exactly failed.

-Liran

> +			}
> +
> +			continue;
> +		}
> +
> 		vcpu_info.pi_desc_addr = __pa(vcpu_to_pi_desc(vcpu));
> 		vcpu_info.vector = irq.vector;
> 
> -- 
> 2.17.1
> 
> 
> 
> 
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