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

[David Woodhouse]

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.

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.

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