Re: [PATCH 2/2] KVM: lapic: Fixup LDR on load in x2apic

[Liran Alon <LIRAN.ALON@ORACLE.COM>]

On 17/11/17 13:52, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>
> In x2apic mode the LDR is fixed based on the ID rather
> than separately loadable like it was before x2.
> When kvm_apic_set_state is called, the base is set, and if
> it has the X2APIC_ENABLE flag set then the LDR is calculated;
> however that value gets overwritten by the memcpy a few lines
> below overwriting it with the value that came from userland.
>
> The symptom is a lack of EOI after loading the state
> (e.g. after a QEMU migration) and is due to the EOI bitmap
> being wrong due to the incorrect LDR.  This was seen with
> a Win2016 guest under Qemu with irqchip=split whose USB mouse
> didn't work after a VM migration.
>
> This corresponds to RH bug:
>    https://urldefense.proofpoint.com/v2/url?u=https-3A__bugzilla.redhat.com_show-5Fbug.cgi-3Fid-3D1502591&d=DwIBaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=BDEsclkj9SNWbZXiuKgX07QVY0LqtwHA13yqtK4wreE&s=MJS_JxKV0dJS6T8qobO29j530xNJLFqgSuRMP8oEiwI&e=
>
> Reported-by: Yiqian Wei <yiwei@redhat.com>
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>   arch/x86/kvm/lapic.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 4991e9e51611..cff55beb0263 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2201,7 +2201,12 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
>   {
>   	if (apic_x2apic_mode(vcpu->arch.apic)) {
>   		u32 *id = (u32 *)(s->regs + APIC_ID);
> +		u32 *ldr = (u32 *)(s->regs + APIC_LDR);
>
> +		if (set) {
> +			/* In x2 the LDR is fixed based on the id */
> +			*ldr = kvm_apic_calc_x2apic_ldr(*id);
> +		}
>   		if (vcpu->kvm->arch.x2apic_format) {
>   			if (*id != vcpu->vcpu_id)
>   				return -EINVAL;
>

I think there is a bug here of not adding the new code in the right 
place. I think diff should be instead:

@@ -2245,6 +2245,7 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
  {
         if (apic_x2apic_mode(vcpu->arch.apic)) {
                 u32 *id = (u32 *)(s->regs + APIC_ID);
+               u32 *ldr = (u32 *)(s->regs + APIC_LDR);

                 if (vcpu->kvm->arch.x2apic_format) {
                         if (*id != vcpu->vcpu_id)
@@ -2255,6 +2256,8 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
                         else
                                 *id <<= 24;
                 }
+
+               *ldr = kvm_apic_calc_x2apic_ldr(*id);
         }

	return 0;

This is because of the x2apic_format hack.
(Fore more info, see commit 3713131345fb ("KVM: x86: add 
KVM_CAP_X2APIC_API")).
Otherwise, you will use a value which can be shifted-left by 24.

-Liran