Re: [PATCH v2 6.1.y 1/3] KVM: nVMX: Add a helper to get highest pending from Posted Interrupt vector

[Sean Christopherson <seanjc@google.com>]

On Fri, Jun 19, 2026, Nicholas Dudar wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> commit d83c36d822be44db4bad0c43bea99c8908f54117 upstream.
> 
> Add a helper to retrieve the highest pending vector given a Posted
> Interrupt descriptor.  While the actual operation is straightforward, it's
> surprisingly easy to mess up, e.g. if one tries to reuse lapic.c's
> find_highest_vector(), which doesn't work with PID.PIR due to the APIC's
> IRR and ISR component registers being physically discontiguous (they're
> 4-byte registers aligned at 16-byte intervals).
> 
> To make PIR handling more consistent with respect to IRR and ISR handling,
> return -1 to indicate "no interrupt pending".
> 
> Cc: stable@vger.kernel.org
> Link: https://lore.kernel.org/r/20240607172609.3205077-2-seanjc@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> [ Nicholas Dudar: backport to 6.1.y. 6.1.y defines struct pi_desc in
>   posted_intr.h and predates the move to <asm/posted_intr.h>, so the helper
>   and the <linux/find.h> include go in posted_intr.h. ]

This is misleading.  The helper is in arch/x86/kvm/vmx/posted_intr.h, even in
upstream.  I don't know if I *intentionally* put the helper in KVM code, but for
for whatever reason, I did.

Commit 699f67512f04 caused a conflict that needed to be resolved in the 6.1
backport, but that didn't have anything to do with needing to re-home the helper.

That only matters because I was going to ask if we'd be better off backporting
asm/posted_intr.h so that the helper would live in it's "proper" location, but
the answer on that front is "no", because it's already there.

FWIW, I got the same conflict resolution, it's just the blurb that's confusing.

> Signed-off-by: Nicholas Dudar <main.kalliope@gmail.com>
> ---
>  arch/x86/kvm/vmx/nested.c      |  5 +++--
>  arch/x86/kvm/vmx/posted_intr.h | 10 ++++++++++
>  2 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index bdc462944..7d8e18dbe 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -12,6 +12,7 @@
>  #include "mmu.h"
>  #include "nested.h"
>  #include "pmu.h"
> +#include "posted_intr.h"
>  #include "sgx.h"
>  #include "trace.h"
>  #include "vmx.h"
> @@ -3818,8 +3819,8 @@ static int vmx_complete_nested_posted_interrupt(struct kvm_vcpu *vcpu)
>  	if (!pi_test_and_clear_on(vmx->nested.pi_desc))
>  		return 0;
>  
> -	max_irr = find_last_bit((unsigned long *)vmx->nested.pi_desc->pir, 256);
> -	if (max_irr != 256) {
> +	max_irr = pi_find_highest_vector(vmx->nested.pi_desc);
> +	if (max_irr > 0) {
>  		vapic_page = vmx->nested.virtual_apic_map.hva;
>  		if (!vapic_page)
>  			goto mmio_needed;
> diff --git a/arch/x86/kvm/vmx/posted_intr.h b/arch/x86/kvm/vmx/posted_intr.h
> index 269920765..88cea0dac 100644
> --- a/arch/x86/kvm/vmx/posted_intr.h
> +++ b/arch/x86/kvm/vmx/posted_intr.h
> @@ -2,6 +2,8 @@
>  #ifndef __KVM_X86_VMX_POSTED_INTR_H
>  #define __KVM_X86_VMX_POSTED_INTR_H
>  
> +#include <linux/find.h>
> +
>  #define POSTED_INTR_ON  0
>  #define POSTED_INTR_SN  1
>  
> @@ -103,4 +105,12 @@ int vmx_pi_update_irte(struct kvm *kvm, unsigned int host_irq,
>  		       uint32_t guest_irq, bool set);
>  void vmx_pi_start_assignment(struct kvm *kvm);
>  
> +static inline int pi_find_highest_vector(struct pi_desc *pi_desc)
> +{
> +	int vec;
> +
> +	vec = find_last_bit((unsigned long *)pi_desc->pir, 256);
> +	return vec < 256 ? vec : -1;
> +}
> +
>  #endif /* __KVM_X86_VMX_POSTED_INTR_H */
> -- 
> 2.34.1
>