linux-kernel - Re: [PATCH 0/5] iommu/vt-d: Consolidate various cache flush ops

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Date:   Wed, 4 Dec 2019 08:27:10 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>
Cc:     baolu.lu@...ux.intel.com, ashok.raj@...el.com,
        jacob.jun.pan@...ux.intel.com, kevin.tian@...el.com,
        Eric Auger <eric.auger@...hat.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] iommu/vt-d: Consolidate various cache flush ops

Hi David,

On 12/3/19 4:49 PM, David Woodhouse wrote:
> On Fri, 2019-11-22 at 11:04 +0800, Lu Baolu wrote:
>> Intel VT-d 3.0 introduces more caches and interfaces for software to
>> flush when it runs in the scalable mode. Currently various cache flush
>> helpers are scattered around. This consolidates them by putting them in
>> the existing iommu_flush structure.
>>
>> /* struct iommu_flush - Intel IOMMU cache invalidation ops
>>   *
>>   * @cc_inv: invalidate context cache
>>   * @iotlb_inv: Invalidate IOTLB and paging structure caches when software
>>   *             has changed second-level tables.
>>   * @p_iotlb_inv: Invalidate IOTLB and paging structure caches when software
>>   *               has changed first-level tables.
>>   * @pc_inv: invalidate pasid cache
>>   * @dev_tlb_inv: invalidate cached mappings used by requests-without-PASID
>>   *               from the Device-TLB on a endpoint device.
>>   * @p_dev_tlb_inv: invalidate cached mappings used by requests-with-PASID
>>   *                 from the Device-TLB on an endpoint device
>>   */
>> struct iommu_flush {
>>          void (*cc_inv)(struct intel_iommu *iommu, u16 did,
>>                         u16 sid, u8 fm, u64 type);
>>          void (*iotlb_inv)(struct intel_iommu *iommu, u16 did, u64 addr,
>>                            unsigned int size_order, u64 type);
>>          void (*p_iotlb_inv)(struct intel_iommu *iommu, u16 did, u32 pasid,
>>                              u64 addr, unsigned long npages, bool ih);
>>          void (*pc_inv)(struct intel_iommu *iommu, u16 did, u32 pasid,
>>                         u64 granu);
>>          void (*dev_tlb_inv)(struct intel_iommu *iommu, u16 sid, u16 pfsid,
>>                              u16 qdep, u64 addr, unsigned int mask);
>>          void (*p_dev_tlb_inv)(struct intel_iommu *iommu, u16 sid, u16 pfsid,
>>                                u32 pasid, u16 qdep, u64 addr,
>>                                unsigned long npages);
>> };
>>
>> The name of each cache flush ops is defined according to the spec section 6.5
>> so that people are easy to look up them in the spec.
> 
> Hm, indirect function calls are quite expensive these days.

Good consideration. Thanks!

> 
> I would have preferred to go in the opposite direction, since surely
> aren't going to have *many* of these implementations. Currently there's
> only one for register-based and one for queued invalidation, right?
> Even if VT-d 3.0 throws an extra version in, I think I'd prefer to take
> out the indirection completely and have an if/then helper.
> 
> Would love to see a microbenchmark of unmap operations before and after
> this patch series with retpoline enabled, to see the effect.

Yes. Need some micro-bench tests to address the concern.

Best regards,
baolu