Re: [Android-virt] [PATCH v5 04/13] ARM: KVM: Memory virtualization setup

[Avi Kivity <avi@redhat.com>]

On 12/12/2011 05:09 PM, Christoffer Dall wrote:
> On Mon, Dec 12, 2011 at 9:40 AM, Avi Kivity <avi@redhat.com> wrote:
> > On 12/11/2011 12:24 PM, Christoffer Dall wrote:
> >> This commit introduces the framework for guest memory management
> >> through the use of 2nd stage translation. Each VM has a pointer
> >> to a level-1 tabled (the pgd field in struct kvm_arch) which is
> >> used for the 2nd stage translations. Entries are added when handling
> >> guest faults (later patch) and the table itself can be allocated and
> >> freed through the following functions implemented in
> >> arch/arm/kvm/arm_mmu.c:
> >>  - kvm_alloc_stage2_pgd(struct kvm *kvm);
> >>  - kvm_free_stage2_pgd(struct kvm *kvm);
> >>
> >> Further, each entry in TLBs and caches are tagged with a VMID
> >> identifier in addition to ASIDs. The VMIDs are managed using
> >> a bitmap and assigned when creating the VM in kvm_arch_init_vm()
> >> where the 2nd stage pgd is also allocated. The table is freed in
> >> kvm_arch_destroy_vm(). Both functions are called from the main
> >> KVM code.
> >>
> >>
> >>  struct kvm_arch {
> >> -     pgd_t *pgd;     /* 1-level 2nd stage table */
> >> +     u32    vmid;    /* The VMID used for the virt. memory system */
> >> +     pgd_t *pgd;     /* 1-level 2nd stage table */
> >> +     u64    vttbr;   /* VTTBR value associated with above pgd and vmid */
> >>  };
> >>
> >
> > I can't say I have a solid grasp here, but my feeling is that vmid needs
> > to be per-vcpu.  Otherwise vcpu 1 can migrate to a cpu that previously
> > ran vcpu 0, and reuse its tlb since they have the same vmid.
> >
> According to the ARM guys, the same rules apply as for ASIDs which
> results into 1 vmid per VM. The vmid is checked along with a
> guest-specific asid, so they would only share TLB entries when running
> the same guest process, which I assume is fine then. Actually
> desirable. But, granted, my SMP knowledge is limited so far, so if
> Marc or Catalin feels like chipping in here...

We need to differentiate in how Linux-as-a-guest acts and how the cpu is
supposed to work.  A guest operating system can theoretically assign the
ASID x to process A running on vcpu 0, and the same ASID x to process B
running on vcpu 1, and be sure that TLB entries don't leak from A to B
since the TLB is cpu local (is that in fact correct?).  Does the ARM
arch allow this, or does it forbid running different contexts on
different vcpus with the same ASID?

-- 
error compiling committee.c: too many arguments to function