Re: [PATCH v4 27/32] KVM: SVM: Update svm->ldr_reg cache even if LDR is "bad"

[Maxim Levitsky <mlevitsk@redhat.com>]

On Sat, 2022-10-01 at 00:59 +0000, Sean Christopherson wrote:
> Update SVM's cache of the LDR even if the new value is "bad".  Leaving
> stale information in the cache can result in KVM missing updates and/or
> invalidating the wrong entry, e.g. if avic_invalidate_logical_id_entry()
> is triggered after a different vCPU has "claimed" the old LDR.
> 
> Fixes: 18f40c53e10f ("svm: Add VMEXIT handlers for AVIC")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/svm/avic.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 2b640c73f447..4b6fc9d64f4d 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -539,23 +539,24 @@ static u32 *avic_get_logical_id_entry(struct kvm_vcpu *vcpu, u32 ldr, bool flat)
>  	return &logical_apic_id_table[index];
>  }
>  
> -static int avic_ldr_write(struct kvm_vcpu *vcpu, u8 g_physical_id, u32 ldr)
> +static void avic_ldr_write(struct kvm_vcpu *vcpu, u8 g_physical_id, u32 ldr)
>  {
>  	bool flat;
>  	u32 *entry, new_entry;
>  
> +	if (!ldr)
> +		return;
> +
The avic_get_logical_id_entry already returns NULL in this case, so I don't think
that this is needed.

>  	flat = kvm_lapic_get_reg(vcpu->arch.apic, APIC_DFR) == APIC_DFR_FLAT;
>  	entry = avic_get_logical_id_entry(vcpu, ldr, flat);
>  	if (!entry)
> -		return -EINVAL;
> +		return;
>  
>  	new_entry = READ_ONCE(*entry);
>  	new_entry &= ~AVIC_LOGICAL_ID_ENTRY_GUEST_PHYSICAL_ID_MASK;
>  	new_entry |= (g_physical_id & AVIC_LOGICAL_ID_ENTRY_GUEST_PHYSICAL_ID_MASK);
>  	new_entry |= AVIC_LOGICAL_ID_ENTRY_VALID_MASK;
>  	WRITE_ONCE(*entry, new_entry);
> -
> -	return 0;
>  }
>  
>  static void avic_invalidate_logical_id_entry(struct kvm_vcpu *vcpu)
> @@ -575,7 +576,6 @@ static void avic_invalidate_logical_id_entry(struct kvm_vcpu *vcpu)
>  
>  static void avic_handle_ldr_update(struct kvm_vcpu *vcpu)
>  {
> -	int ret = 0;
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  	u32 ldr = kvm_lapic_get_reg(vcpu->arch.apic, APIC_LDR);
>  	u32 id = kvm_xapic_id(vcpu->arch.apic);
> @@ -589,11 +589,8 @@ static void avic_handle_ldr_update(struct kvm_vcpu *vcpu)
>  
>  	avic_invalidate_logical_id_entry(vcpu);
>  
> -	if (ldr)
> -		ret = avic_ldr_write(vcpu, id, ldr);
> -
> -	if (!ret)
> -		svm->ldr_reg = ldr;
> +	svm->ldr_reg = ldr;
> +	avic_ldr_write(vcpu, id, ldr);
>  }
>  
>  static void avic_handle_dfr_update(struct kvm_vcpu *vcpu)


With the nitpick fixed:

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>


Best regards,
	Maxim Levitsky <mlevitsk@redhat.com>