Re: [PATCH] KVM: SVM: Fix x2apic logical cluster mode decoding and sanity check

[Suthikulpanit, Suravee]

On 9/13/2022 12:29 AM, Sean Christopherson wrote:
> On Mon, Sep 12, 2022, Suravee Suthikulpanit wrote:
> > When sending IPI in the X2APIC logical cluster mode, the destination
> > APIC ID is encoded as:
> >
> >    * Cluster ID = ICRH[31:16]
> >    * Logical ID = ICRH[15:0]
> >
> > Current logic incorrectly decode the ICRH, which causes VM running
> > with x2AVIC support to fail to boot. Therefore, fix the decoding logic.
>
> There are already patches pending[1][2] for the x2AVIC bugs.
>
> [1] https://lore.kernel.org/all/20220903002254.2411750-9-seanjc@xxxxxxxxxx
> [2] https://lore.kernel.org/all/20220903002254.2411750-10-seanjc@xxxxxxxxxx

Ok. Thanks for the pointer.

> > The commit 603ccef42ce9 ("KVM: x86: SVM: fix avic_kick_target_vcpus_fast")
> > also added a check for multiple logical destinations before using
> > the fast-path. However, the same logic is already existed prior to
> > the commit. Therefore, remove redundant checking logic.
> >
> > Fixes: 603ccef42ce9 ("KVM: x86: SVM: fix avic_kick_target_vcpus_fast")
> > Cc: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> > ---
> >   arch/x86/kvm/svm/avic.c | 19 ++++---------------
> >   1 file changed, 4 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > index 6919dee69f18..45ab49d1f0b8 100644
> > --- a/arch/x86/kvm/svm/avic.c
> > +++ b/arch/x86/kvm/svm/avic.c
> > @@ -378,8 +378,8 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
> >   
> >   		if (apic_x2apic_mode(source)) {
> >   			/* 16 bit dest mask, 16 bit cluster id */
> > -			bitmap = dest & 0xFFFF0000;
> > -			cluster = (dest >> 16) << 4;
> > +			bitmap = dest & 0xffff;
> > +			cluster = (dest & 0xffff0000) >> 16;
> >   		} else if (kvm_lapic_get_reg(source, APIC_DFR) == APIC_DFR_FLAT) {
> >   			/* 8 bit dest mask*/
> >   			bitmap = dest;
> > @@ -387,7 +387,7 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
> >   		} else {
> >   			/* 4 bit desk mask, 4 bit cluster id */
> >   			bitmap = dest & 0xF;
> > -			cluster = (dest >> 4) << 2;
> > +			cluster = (dest & 0xf0) >> 4;
>
> This is wrong and unrelated.  The cluster needs to be shifted back by 2, i.e. multiplied
> by 4, to leap over each cluster of 4 IDs.

Ah, I missed the logic that was using the _cluster_ value below.

Thanks,
Suravee