Re: [Qemu-ppc] [PATCH] KVM: PPC: Book3S PR: only call slbmte for valid SLB entries

[David Gibson]

On Fri, Sep 22, 2017 at 11:34:29AM +0200, Greg Kurz wrote:
> Userland passes an array of 64 SLB descriptors to KVM_SET_SREGS,
> some of which are valid (ie, SLB_ESID_V is set) and the rest are
> likely all-zeroes (with QEMU at least).
> 
> Each of them is then passed to kvmppc_mmu_book3s_64_slbmte(), which
> assumes to find the SLB index in the 3 lower bits of its rb argument.
> When passed zeroed arguments, it happily overwrites the 0th SLB entry
> with zeroes. This is exactly what happens while doing live migration
> with QEMU when the destination pushes the incoming SLB descriptors to
> KVM PR. When reloading the SLBs at the next synchronization, QEMU first
> clears its SLB array and only restore valid ones, but the 0th one is
> now gone and we cannot access the corresponding memory anymore:
> 
> (qemu) x/x $pc
> c0000000000b742c: Cannot access memory
> 
> To avoid this, let's filter out non-valid SLB entries, like we
> already do for Book3S HV.
> 
> Signed-off-by: Greg Kurz <address@hidden>

This seems like a good idea, but to make it fully correct, don't we
also need to fully flush the SLB before inserting the new entries.

> ---
>  arch/powerpc/kvm/book3s_pr.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
> index 3beb4ff469d1..cb6894e55f97 100644
> --- a/arch/powerpc/kvm/book3s_pr.c
> +++ b/arch/powerpc/kvm/book3s_pr.c
> @@ -1328,8 +1328,10 @@ static int kvm_arch_vcpu_ioctl_set_sregs_pr(struct 
> kvm_vcpu *vcpu,
>       vcpu3s->sdr1 = sregs->u.s.sdr1;
>       if (vcpu->arch.hflags & BOOK3S_HFLAG_SLB) {
>               for (i = 0; i < 64; i++) {
> -                     vcpu->arch.mmu.slbmte(vcpu, 
> sregs->u.s.ppc64.slb[i].slbv,
> -                                                 
> sregs->u.s.ppc64.slb[i].slbe);
> +                     u64 rb = sregs->u.s.ppc64.slb[i].slbe;
> +                     u64 rs = sregs->u.s.ppc64.slb[i].slbv;
> +                     if (rb & SLB_ESID_V)
> +                             vcpu->arch.mmu.slbmte(vcpu, rs, rb);
>               }
>       } else {
>               for (i = 0; i < 16; i++) {
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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