Re: [PATCH v3 06/12] KVM: arm/arm64: Helper to register a new redistributor region

Christoffer Dall
2018-04-26T10:04:25Z

On Thu, Apr 26, 2018 at 09:32:49AM +0200, Auger Eric wrote:
> Hi Christoffer,
>
> On 04/24/2018 06:47 PM, Christoffer Dall wrote:
> > On Fri, Apr 13, 2018 at 10:20:52AM +0200, Eric Auger wrote:
> >> We introduce a new helper that creates and inserts a new redistributor
> >> region into the rdist region list. This helper both handles the case
> >> where the redistributor region size is known at registration time
> >> and the legacy case where it is not (eventually depending on the number
> >> of online vcpus). Depending on pfns, we perform all the possible checks
> >> that we can do:
> >>
> >> - end of memory crossing
> >> - incorrect alignment of the base address
> >> - collision with distributor region if already defined
> >> - collision with already registered rdist regions
> >> - check of the new index
> >>
> >> Rdist regions must be inserted by increasing order of indices. Indices
> >> must be contiguous.
> >>
> >> We also introduce vgic_v3_rdist_region_from_index() which will be used
> >> from the vgic kvm-device, later on.
> >>
> >> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >> ---
> >> | 95 +++++++++++++++++++++++++++++++++-------
> >> virt/kvm/arm/vgic/vgic-v3.c | 29 ++++++++++++
> >> virt/kvm/arm/vgic/vgic.h | 14 ++++++
> >> 3 files changed, 122 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> >> index ce5c927..5273fb8 100644
> >> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
> >> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> >> @@ -680,14 +680,66 @@ static int vgic_register_all_redist_iodevs(struct kvm *kvm)
> >> return ret;
> >> }
> >>
> >> -int vgic_v3_set_redist_base(struct kvm *kvm, u64 addr)
> >> +/**
> >> + * vgic_v3_insert_redist_region - Insert a new redistributor region
> >> + *
> >> + * Performs various checks before inserting the rdist region in the list.
> >> + * Those tests depend on whether the size of the rdist region is known
> >> + * (ie. count != 0). The list is sorted by rdist region index.
> >> + *
> >> + * @kvm: kvm handle
> >> + * @index: redist region index
> >> + * @base: base of the new rdist region
> >> + * @count: number of redistributors the region is made of (of 0 in the old style
> >> + * single region, whose size is induced from the number of vcpus)
> >> + *
> >> + * Return 0 on success, < 0 otherwise
> >> + */
> >> +static int vgic_v3_insert_redist_region(struct kvm *kvm, uint32_t index,
> >> + gpa_t base, uint32_t count)
> >> {
> >> - struct vgic_dist *vgic = &kvm->arch.vgic;
> >> + struct vgic_dist *d = &kvm->arch.vgic;
> >> struct vgic_redist_region *rdreg;
> >> + struct list_head *rd_regions = &d->rd_regions;
> >> + struct list_head *last = rd_regions->prev;
> >> +
> >
> > nit: extra blank line?
> >
> >> + gpa_t new_start, new_end;
> >> + size_t size = count * KVM_VGIC_V3_REDIST_SIZE;
> >> int ret;
> >>
> >> - /* vgic_check_ioaddr makes sure we don't do this twice */
> >> - if (!list_empty(&vgic->rd_regions))
> >> + /* single rdist region already set ?*/
> >> + if (!count && !list_empty(rd_regions))
> >> + return -EINVAL;
> >> +
> >> + /* cross the end of memory ? */
> >> + if (base + size < base)
> >> + return -EINVAL;
> >
> > what is the size of memory? This seems to check for a gpa_t overflow,
> > but not againt the IPA space of the VM...
> Yes it checks for a gpa_t overflow. This check is currently done in
> vgic_v3_check_base() for dist and redist region and I replicated it.

fair enough, the comment is a bit misleading though. We could also
consider checking against KVM_PHYS_SHIFT.

> >
> >> +
> >> + if (list_empty(rd_regions)) {
> >> + if (index != 0)
> >> + return -EINVAL;
> >
> > note, I think this can be simplified if we can rid of the index.
> So I eventually keep the index.

Yes.

> >
> >> + } else {
> >> + rdreg = list_entry(last, struct vgic_redist_region, list);
> >
> > you can use list_last_entry here and get rid of the 'last' temporary
> > variable above.
> definitively, thank you for the nit.
> >
> >> + if (index != rdreg->index + 1)
> >> + return -EINVAL;
> >> +
> >> + /* Cannot add an explicitly sized regions after legacy region */
> >> + if (!rdreg->count)
> >> + return -EINVAL;
> >> + }
> >> +
> >> + /*
> >> + * collision with already set dist region ?
> >> + * this assumes we know the size of the new rdist region (pfns != 0)
> >> + * otherwise we can only test this when all vcpus are registered
> >> + */
> >
> > I don't really understand this commentary... :(
> I meant we cannot perform the check below if we are inserting a unique
> legacy rdist region (old API), whose size is not explicitly set but
> induced from the number of online vcpus.
>

ok, given the complexity of the logic below, I think you should just
explain it:
/*
* For legacy single-region redistributor regions (!count),
* check that the redistributor region does not overlap with the
* distributor's address space.
*/

> >
> >> + if (!count && !IS_VGIC_ADDR_UNDEF(d->vgic_dist_base) &&
> >> + (!(d->vgic_dist_base + KVM_VGIC_V3_DIST_SIZE <= base)) &&
> >> + (!(base + size <= d->vgic_dist_base)))
> >> + return -EINVAL;
> >
> > Can't you call vgic_v3_check_base() here instead?
> no I can't because vgic_v3_check_base() currently only works with the
> unique legacy rdist region. There, redist_size is
> atomic_read(&kvm->online_vcpus) * KVM_VGIC_V3_REDIST_SIZE.

Hmmm, ok. I'm not completely clear if that can be reworked to be reused
or not, but perhaps you could just introduce a primitive ?

static bool redist_overlaps_dist(struct kvm *kvm, gpa_t rd_base, size_t rd_size);

> >
> >> +
> >> + /* collision with any other rdist region? */
> >> + if (vgic_v3_rdist_overlap(kvm, base, size))
> >> return -EINVAL;
> >>
> >> rdreg = kzalloc(sizeof(*rdreg), GFP_KERNEL);
> >> @@ -696,17 +748,32 @@ int vgic_v3_set_redist_base(struct kvm *kvm, u64 addr)
> >>
> >> rdreg->base = VGIC_ADDR_UNDEF;
> >>
> >> - ret = vgic_check_ioaddr(kvm, &rdreg->base, addr, SZ_64K);
> >> + ret = vgic_check_ioaddr(kvm, &rdreg->base, base, SZ_64K);
> >> if (ret)
> >> - goto out;
> >> + goto free;
> >>
> >> - rdreg->base = addr;
> >> - if (!vgic_v3_check_base(kvm)) {
> >> - ret = -EINVAL;
> >> - goto out;
> >> - }
> >> + rdreg->base = base;
> >> + rdreg->count = count;
> >> + rdreg->free_index = 0;
> >> + rdreg->index = index;
> >>
> >> - list_add(&rdreg->list, &vgic->rd_regions);
> >> + new_start = base;
> >> + new_end = base + size - 1;
> >
> > What are these variables used for?
> Hum reminder from an old version :-(
> >
> >> +
> >> + list_add_tail(&rdreg->list, rd_regions);
> >> + return 0;
> >> +free:
> >> + kfree(rdreg);
> >> + return ret;
> >> +}
> >> +
> >> +int vgic_v3_set_redist_base(struct kvm *kvm, u64 addr)
> >> +{
> >> + int ret;
> >> +
> >> + ret = vgic_v3_insert_redist_region(kvm, 0, addr, 0);
> >> + if (ret)
> >> + return ret;
> >>
> >> /*
> >> * Register iodevs for each existing VCPU. Adding more VCPUs
> >> @@ -717,10 +784,6 @@ int vgic_v3_set_redist_base(struct kvm *kvm, u64 addr)
> >> return ret;
> >>
> >> return 0;
> >> -
> >> -out:
> >> - kfree(rdreg);
> >> - return ret;
> >> }
> >>
> >> int vgic_v3_has_attr_regs(struct kvm_device *dev, struct kvm_device_attr *attr)
> >> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
> >> index 820012a..dbcba5f 100644
> >> --- a/virt/kvm/arm/vgic/vgic-v3.c
> >> +++ b/virt/kvm/arm/vgic/vgic-v3.c
> >> @@ -410,6 +410,21 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)
> >> return 0;
> >> }
> >>
> >> +/* return true if there is an overlap between any rdist */
> >
> > Checks if base+size overlaps with any existing redistributor.
> >
> >> +bool vgic_v3_rdist_overlap(struct kvm *kvm, gpa_t base, size_t size)
> >> +{
> >> + struct vgic_dist *d = &kvm->arch.vgic;
> >> + struct vgic_redist_region *rdreg;
> >> +
> >> + list_for_each_entry(rdreg, &d->rd_regions, list) {
> >> + if ((base + size <= rdreg->base) ||
> >> + (rdreg->base + vgic_v3_rd_region_size(kvm, rdreg) <= base))
> >> + continue;
> >> + return true;
> >
> > can you invert the check above and return false instead of the continue?
> >
> > (DeMorgan's law should be handy here.)
> sure
> >
> >> + }
> >> + return false;
> >> +}
> >> +
> >> /*
> >> * Check for overlapping regions and for regions crossing the end of memory
> >> * for base addresses which have already been set.
> >> @@ -461,6 +476,20 @@ struct vgic_redist_region *vgic_v3_rdist_free_slot(struct list_head *rd_regions)
> >> return NULL;
> >> }
> >>
> >> +struct vgic_redist_region *vgic_v3_rdist_region_from_index(struct kvm *kvm,
> >> + uint32_t index)
> >> +{
> >> + struct list_head *rd_regions = &kvm->arch.vgic.rd_regions;
> >> + struct vgic_redist_region *rdreg;
> >> +
> >> + list_for_each_entry(rdreg, rd_regions, list) {
> >> + if (rdreg->index == index)
> >> + return rdreg;
> >> + }
> >
> > if this ends up being a common operation, we could allocate an array of
> > pointers for constant-time lookup instead. Let's hope it's not too
> > common.
> This is only used when reading the characteristics of a redist region
> from userspace so I don't think we care.
>

ok, fine.

Thanks,
-Christoffer