Re: [PATCH V2] intel_iommu: refine iotlb hash calculation

["Alex Bennée" <alex.bennee@linaro.org>]

Jason Wang <jasowang@redhat.com> writes:

> On Wed, Apr 12, 2023 at 4:43 PM Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>>
>> Jason Wang <jasowang@redhat.com> writes:
>>
>> > Commit 1b2b12376c8 ("intel-iommu: PASID support") takes PASID into
>> > account when calculating iotlb hash like:
>> >
>> > static guint vtd_iotlb_hash(gconstpointer v)
>> > {
>> >     const struct vtd_iotlb_key *key = v;
>> >
>> >     return key->gfn | ((key->sid) << VTD_IOTLB_SID_SHIFT) |
>> >            (key->level) << VTD_IOTLB_LVL_SHIFT |
>> >            (key->pasid) << VTD_IOTLB_PASID_SHIFT;
>> > }
>> >
>> > This turns out to be problematic since:
>> >
>> > - the shift will lose bits if not converting to uint64_t
>> > - level should be off by one in order to fit into 2 bits
>> > - VTD_IOTLB_PASID_SHIFT is 30 but PASID is 20 bits which will waste
>> >   some bits
>> > - the hash result is uint64_t so we will lose bits when converting to
>> >   guint
>> >
>> > So this patch fixes them by
>> >
>> > - converting the keys into uint64_t before doing the shift
>> > - off level by one to make it fit into two bits
>> > - change the sid, lvl and pasid shift to 26, 42 and 44 in order to
>> >   take the full width of uint64_t
>> > - perform an XOR to the top 32bit with the bottom 32bit for the final
>> >   result to fit guint
>> >
>> > Fixes: Coverity CID 1508100
>> > Fixes: 1b2b12376c8 ("intel-iommu: PASID support")
>> > Signed-off-by: Jason Wang <jasowang@redhat.com>
>> > ---
>> > Changes since V1:
>> > - perform XOR to avoid losing bits when converting to gint
>> > ---
>> >  hw/i386/intel_iommu.c          | 9 +++++----
>> >  hw/i386/intel_iommu_internal.h | 6 +++---
>> >  2 files changed, 8 insertions(+), 7 deletions(-)
>> >
>> > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
>> > index a62896759c..94d52f4205 100644
>> > --- a/hw/i386/intel_iommu.c
>> > +++ b/hw/i386/intel_iommu.c
>> > @@ -64,8 +64,8 @@ struct vtd_as_key {
>> >  struct vtd_iotlb_key {
>> >      uint64_t gfn;
>> >      uint32_t pasid;
>> > -    uint32_t level;
>> >      uint16_t sid;
>> > +    uint8_t level;
>> >  };
>> >
>> >  static void vtd_address_space_refresh_all(IntelIOMMUState *s);
>> > @@ -221,10 +221,11 @@ static gboolean vtd_iotlb_equal(gconstpointer v1, gconstpointer v2)
>> >  static guint vtd_iotlb_hash(gconstpointer v)
>> >  {
>> >      const struct vtd_iotlb_key *key = v;
>> > +    uint64_t hash64 = key->gfn | ((uint64_t)(key->sid) << VTD_IOTLB_SID_SHIFT) |
>> > +        (uint64_t)(key->level - 1) << VTD_IOTLB_LVL_SHIFT |
>> > +        (uint64_t)(key->pasid) << VTD_IOTLB_PASID_SHIFT;
>> >
>> > -    return key->gfn | ((key->sid) << VTD_IOTLB_SID_SHIFT) |
>> > -           (key->level) << VTD_IOTLB_LVL_SHIFT |
>> > -           (key->pasid) << VTD_IOTLB_PASID_SHIFT;
>> > +    return (guint)((hash64 >> 32) ^ (hash64 & 0xffffffffU));
>>
>> Have you measured the distribution this hash gives you? Otherwise
>> consider using the qemu_xxhash() functions to return a well distributed
>> 32 bit hash value.
>
> It depends on a lot of factors and so it won't be even because the
> individuals keys are not evenly distributed:
>
> - gfn depends on guest DMA subsystems
> - level depends on when huge pages are used
> - pasid depends on whether PASID is being used
>
> I'm ok to switch to use qemu_xxhash() if everyone agree. Or if as
> Peter said, if it has been dealt with glibc, maybe it's not worth to
> bother.

Yeah I missed that glibs default hash functions where pretty basic. I
think you can ignore my suggestion.

>
> Thanks
>
>>
>> --
>> Alex Bennée
>> Virtualisation Tech Lead @ Linaro
>>


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro