Re: [PATCH v4 17/22] KVM: arm64: ITS: Add infrastructure for table lookup

Marc Zyngier
2017-04-09T10:34:01Z

On Mon, Mar 27 2017 at 10:31:07 AM, Eric Auger <eric.auger@redhat.com> wrote:
> Add a generic lookup_table() helper whose role consists in
> scanning a contiguous table located in guest RAM and applying
> a callback on each entry. Entries can be handled as linked lists
> since the callback may return an offset to the next entry and
> also tell that an entry is the last one.
>
> Helper functions also are added to compute the device/event ID
> offset to the next DTE/ITE.
>
> compute_next_devid_offset, compute_next_eventid_offset and
> lookup_table will become static in subsequent patches
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>
> ---
>
> v3 -> v4:
> - remove static to avoid compilation warning
> - correct size computation in looup_table()
> - defines now encode the number of bits used for devid and eventid offsets
> - use BIT() - 1 to encode the max offets
> ---
> virt/kvm/arm/vgic/vgic-its.c | 107 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 107 insertions(+)
>
> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> index 7364b7d..8eaeba4 100644
> --- a/virt/kvm/arm/vgic/vgic-its.c
> +++ b/virt/kvm/arm/vgic/vgic-its.c
> @@ -184,6 +184,8 @@ static struct its_ite *find_ite(struct vgic_its *its, u32 device_id,
> #define VITS_ESZ 8
> #define VITS_TYPER_IDBITS 16
> #define VITS_TYPER_DEVBITS 16
> +#define VITS_DTE_MAX_DEVID_OFFSET (BIT(14) - 1)
> +#define VITS_ITE_MAX_EVENTID_OFFSET (BIT(16) - 1)
>
> /*
> * Finds and returns a collection in the ITS collection table.
> @@ -1686,6 +1688,111 @@ int vgic_its_attr_regs_access(struct kvm_device *dev,
> return ret;
> }
>
> +u32 compute_next_devid_offset(struct list_head *h, struct its_device *dev)
> +{
> + struct list_head *e = &dev->dev_list;
> + struct its_device *next;
> + u32 next_offset;
> +
> + if (e->next == h)
> + return 0;
> + next = list_entry(e->next, struct its_device, dev_list);
> + next_offset = next->device_id - dev->device_id;
> +
> + return min_t(u32, next_offset, VITS_DTE_MAX_DEVID_OFFSET);
> +}
> +
> +u32 compute_next_eventid_offset(struct list_head *h, struct its_ite *ite)
> +{
> + struct list_head *e = &ite->ite_list;
> + struct its_ite *next;
> + u32 next_offset;
> +
> + if (e->next == h)
> + return 0;
> + next = list_entry(e->next, struct its_ite, ite_list);
> + next_offset = next->event_id - ite->event_id;
> +
> + return min_t(u32, next_offset, VITS_ITE_MAX_EVENTID_OFFSET);
> +}
> +
> +/**
> + * entry_fn_t - Callback called on a table entry restore path
> + * @its: its handle
> + * @id: id of the entry
> + * @addr: kernel VA of the entry
> + * @opaque: pointer to an opaque data
> + * @next_offset: minimal ID offset to the next entry. 0 if this
> + * entry is the last one, 1 if the entry is invalid, >= 1 if an
> + * entry's next_offset field was truly decoded
> + *
> + * Return: < 0 on error, 0 otherwise
> + */
> +typedef int (*entry_fn_t)(struct vgic_its *its, u32 id, void *addr,
> + void *opaque, u32 *next_offset);
> +
> +/**
> + * lookup_table - scans a contiguous table in guest RAM and applies a function
> + * to each entry
> + *
> + * @its: its handle
> + * @base: base gpa of the table
> + * @size: size of the table in bytes
> + * @esz: entry size in bytes
> + * @start_id: first entry's ID
> + * @fn: function to apply on each entry
> + *
> + * Return: < 0 on error, 1 if last element identified, 0 otherwise
> + */
> +int lookup_table(struct vgic_its *its, gpa_t base, int size, int esz,
> + int start_id, entry_fn_t fn, void *opaque)
> +{
> + gpa_t gpa = base, top = base + size - 1;
> + unsigned long len = size;
> + int ret, seg, offset = offset_in_page(gpa);
> + gfn_t gfn = gpa >> PAGE_SHIFT;
> + u32 id = start_id + (gpa - base)/esz;
> + struct kvm *kvm = its->dev->kvm;
> + kvm_pfn_t pfn;
> +
> + while ((seg = next_segment(len, offset)) != 0) {
> + u32 next_offset;
> + bool writeable;
> + void *addr;
> +
> + pfn = gfn_to_pfn_prot(kvm, gfn, true, &writeable);
> + if (is_error_noslot_pfn(pfn))
> + return -EINVAL;
> + addr = phys_to_virt((u64)pfn << PAGE_SHIFT);
> + addr += offset;

Why do we use the kernel mapping here? I'd have expected the use of
kvm_read_guest and co rather than directly accessing the memory. What
locks do we hold to ensure that the page cannot be evicted behind our
back?

> +
> + while (seg > 0) {
> + size_t byte_offset;
> +
> + ret = fn(its, id, addr, opaque, &next_offset);
> + if (ret < 0 || (!ret && !next_offset))
> + goto out;
> +
> + byte_offset = next_offset * esz;
> +
> + id += next_offset;
> + gpa += byte_offset;
> + addr += byte_offset;
> + seg -= byte_offset;
> + }
> + kvm_release_pfn_clean(pfn);
> +
> + len = top - gpa + 1;
> + offset = offset_in_page(gpa);
> + gfn = gpa >> PAGE_SHIFT;
> + id = start_id + (gpa - base)/esz;
> + }
> + return 0;
> +out:
> + kvm_release_pfn_clean(pfn);
> + return (ret < 0 ? ret : 1);
> +}
> +
> /**
> * vgic_its_flush_pending_tables - Flush the pending tables into guest RAM
> */

Thanks,

M.
--
Jazz is not dead. It just smells funny.