Re: [PATCH v5 16/22] KVM: arm64: vgic-its: Add infrastructure for table lookup

[Christoffer Dall <christoffer.dall@linaro.org>]

On Thu, Apr 27, 2017 at 09:24:57PM +0200, Auger Eric wrote:
> Hi,
> 
> On 27/04/2017 20:06, Christoffer Dall wrote:
> > On Fri, Apr 14, 2017 at 12:15:28PM +0200, Eric Auger 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>
> >>
> >> ---
> >> v4 -> v5:
> >> - use kvm_read_guest
> >>
> >> 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 | 93 ++++++++++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 93 insertions(+)
> >>
> >> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> >> index 56c5123..c22b35d 100644
> >> --- a/virt/kvm/arm/vgic/vgic-its.c
> >> +++ b/virt/kvm/arm/vgic/vgic-its.c
> >> @@ -195,6 +195,8 @@ static struct its_ite *find_ite(struct vgic_its *its, u32 device_id,
> >>  
> >>  #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.
> >> @@ -1674,6 +1676,97 @@ 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
> >> + * @entry: pointer to 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 *entry,
> >> +			  void *opaque, u32 *next_offset);
> >> +
> >> +/**
> >> + * lookup_table - scan 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
> > 
> > I'm a little confused.  Why is this not 0?
> Because this is meant to be called on a second-level table as well
> (device table). In that case the start ID != 0.

Ah, ok, makes sense.  You could use the comment to explain that:
  @start_id: The ID of the first entry in the table
             (non-zero for second-level tables)

> > 
> >> + * @fn: function to apply on each entry
> >> + *
> >> + * Return: < 0 on error, 1 if last element identified, 0 otherwise
> > 
> > How can you scan the entire table and not find the last element?
> This happens on 2d level tables. The last element might be in another 2d
> level table.

Ah, I think you should explain this so that the intended use is clear.

Thanks,
-Christoffer