Re: [PATCH v1 02/15] intel_iommu: Optimize context entry cache utilization

[Eric Auger <eric.auger@redhat.com>]

On 6/18/25 4:10 AM, Duan, Zhenzhong wrote:
> Hi Eric,
>
>> -----Original Message-----
>> From: Eric Auger <eric.auger@redhat.com>
>> Subject: Re: [PATCH v1 02/15] intel_iommu: Optimize context entry cache
>> utilization
>>
>> Hi Zhenzhong,
>>
>> On 6/6/25 12:04 PM, Zhenzhong Duan wrote:
>>> There are many call sites referencing context entry by calling
>>> vtd_dev_to_context_entry() which will traverse the DMAR table.
>>>
>>> In most cases we can use cached context entry in vtd_as->context_cache_entry
>>> except when its entry is stale. Currently only global and domain context
>>> invalidation stale it.
>>>
>>> So introduce a helper function vtd_as_to_context_entry() to fetch from cache
>>> before trying with vtd_dev_to_context_entry().
>>>
>>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
>>> ---
>>>  hw/i386/intel_iommu.c | 36 +++++++++++++++++++++++-------------
>>>  1 file changed, 23 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
>>> index f0b1f90eff..a2f3250724 100644
>>> --- a/hw/i386/intel_iommu.c
>>> +++ b/hw/i386/intel_iommu.c
>>> @@ -1597,6 +1597,22 @@ static int
>> vtd_dev_to_context_entry(IntelIOMMUState *s, uint8_t bus_num,
>>>      return 0;
>>>  }
>>>
>>> +static int vtd_as_to_context_entry(VTDAddressSpace *vtd_as,
>> VTDContextEntry *ce)
>>> +{
>>> +    IntelIOMMUState *s = vtd_as->iommu_state;
>>> +    uint8_t bus_num = pci_bus_num(vtd_as->bus);
>>> +    uint8_t devfn = vtd_as->devfn;
>>> +    VTDContextCacheEntry *cc_entry = &vtd_as->context_cache_entry;
>>> +
>>> +    /* Try to fetch context-entry from cache first */
>>> +    if (cc_entry->context_cache_gen == s->context_cache_gen) {
>>> +        *ce = cc_entry->context_entry;
>>> +        return 0;
>>> +    } else {
>>> +        return vtd_dev_to_context_entry(s, bus_num, devfn, ce);
>>> +    }
>>> +}
>>> +
>> While the patch looks good to me can't you use the helper also in
>> vtd_do_iommu_translate()?
>> See " /* Try to fetch context-entry from cache first */"
> It can, but it finally calls into vtd_dev_to_context_entry() so we can call vtd_dev_to_context_entry() directly.
> I will drop this patch following Yi's suggestion.
OK

Cheers

Eric
>
> Thanks
> Zhenzhong