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    SubjectRe: [PATCH 06/11] Inject asynchronous page fault into a guest if page is swapped out.
    On 11/01/2009 01:56 PM, Gleb Natapov wrote:
    > If guest access swapped out memory do not swap it in from vcpu thread
    > context. Setup slow work to do swapping and send async page fault to
    > a guest.
    >
    > Allow async page fault injection only when guest is in user mode since
    > otherwise guest may be in non-sleepable context and will not be able to
    > reschedule.
    >

    That loses us page cache accesses, which may be the majority of accesses
    in some workloads.

    If we allow the guest to ignore a fault, and ensure that a second access
    to an apf page from the same vcpu doesn't trigger another apf, we can
    simply ignore the apf in a guest when we can't schedule.

    Probably best done with an enable bit for kernel-mode apfs.

    > Signed-off-by: Gleb Natapov<gleb@redhat.com>
    > ---
    > arch/x86/include/asm/kvm_host.h | 20 +++
    > arch/x86/kvm/mmu.c | 243 ++++++++++++++++++++++++++++++++++++++-
    > arch/x86/kvm/mmutrace.h | 60 ++++++++++
    > arch/x86/kvm/paging_tmpl.h | 16 +++-
    > arch/x86/kvm/x86.c | 22 +++-
    >

    Much of the code is generic, please move it to virt/kvm.

    > +static void async_pf_execute(struct slow_work *work)
    > +{
    > + struct page *page[1];
    >

    No need to make it an array, just pass its address.

    > + struct kvm_mmu_async_pf *apf =
    > + container_of(work, struct kvm_mmu_async_pf, work);
    > + wait_queue_head_t *q =&apf->vcpu->wq;
    > +
    > + might_sleep();
    > +
    > + down_read(&apf->mm->mmap_sem);
    > + get_user_pages(current, apf->mm, apf->addr, 1, 1, 0, page, NULL);
    > + up_read(&apf->mm->mmap_sem);
    > +
    > + spin_lock(&apf->vcpu->arch.mmu_async_pf_lock);
    > + list_add_tail(&apf->link,&apf->vcpu->arch.mmu_async_pf_done);
    > + apf->page = page[0];
    > + spin_unlock(&apf->vcpu->arch.mmu_async_pf_lock);
    > +
    > + trace_kvm_mmu_async_pf_executed(apf->addr, apf->page, apf->token,
    > + apf->gva);
    >

    _completed, but maybe better placed in vcpu context.

    > +
    > +static bool can_do_async_pf(struct kvm_vcpu *vcpu)
    > +{
    > + struct kvm_segment kvm_seg;
    > +
    > + if (!vcpu->arch.pv_shm ||
    > + !(vcpu->arch.pv_shm->features& KVM_PV_SHM_FEATURES_ASYNC_PF) ||
    > + kvm_event_needs_reinjection(vcpu))
    > + return false;
    > +
    > + kvm_get_segment(vcpu,&kvm_seg, VCPU_SREG_CS);
    > +
    > + /* is userspace code? TODO check VM86 mode */
    > + return !!(kvm_seg.selector& 3);
    >

    There's a ->get_cpl() which is slightly faster. Note vm86 is perfectly
    fine for async pf.

    > +static int setup_async_pf(struct kvm_vcpu *vcpu, gva_t gva, gfn_t gfn)
    > +{
    > + struct kvm_mmu_async_pf *work;
    > +
    > + /* setup slow work */
    > +
    > + /* do alloc atomic since if we are going to sleep anyway we
    > + may as well sleep faulting in page */
    > + work = kmem_cache_zalloc(mmu_async_pf_cache, GFP_ATOMIC);
    > + if (!work)
    > + return 0;
    > +
    > + atomic_set(&work->used, 1);
    > + work->page = NULL;
    > + work->vcpu = vcpu;
    > + work->gva = gva;
    > + work->addr = gfn_to_hva(vcpu->kvm, gfn);
    > + work->token = (vcpu->arch.async_pf_id++<< 12) | vcpu->vcpu_id;
    >

    The shift truncates async_pf_id.

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



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