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Message-ID: <20171013110243.1ed895f7@t450s.home>
Date: Fri, 13 Oct 2017 11:02:43 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Joerg Roedel <joro@...tes.org>
Cc: iommu@...ts.linux-foundation.org,
Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 4/4] vfio/type1: Gather TLB-syncs and pages to unpin
On Fri, 13 Oct 2017 16:40:13 +0200
Joerg Roedel <joro@...tes.org> wrote:
> From: Joerg Roedel <jroedel@...e.de>
>
> After every unmap VFIO unpins the pages that where mapped by
> the IOMMU. This requires an IOTLB flush after every unmap
> and puts a high load on the IOMMU hardware and the device
> TLBs.
>
> Gather up to 32 ranges to flush and unpin and do the IOTLB
> flush once for all these ranges. This significantly reduces
> the number of IOTLB flushes in the unmapping path.
>
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
> drivers/vfio/vfio_iommu_type1.c | 106 ++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 101 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 2b1e81f..86fc1da 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -107,6 +107,92 @@ struct vfio_pfn {
>
> static int put_pfn(unsigned long pfn, int prot);
>
> +static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
> + unsigned long pfn, long npage,
> + bool do_accounting);
> +
> +#define GATHER_ENTRIES 32
What heuristics make us arrive at this number and how would we evaluate
changing it in the future? Ideally we'd only want to do a single
flush, but we can't unpin pages until after the iommu sync and we need
the iommu to track iova-phys mappings, so it's a matter of how much do
we want to allocate to buffer those translations. I wonder if a cache
pool would help here, but this is probably fine for a first pass with
some comment about this trade-off and why 32 was chosen.
> +
> +/*
> + * Gather TLB flushes before unpinning pages
> + */
> +struct vfio_gather_entry {
> + dma_addr_t iova;
> + phys_addr_t phys;
> + size_t size;
> +};
> +
> +struct vfio_gather {
> + unsigned fill;
> + struct vfio_gather_entry entries[GATHER_ENTRIES];
> +};
> +
> +/*
> + * The vfio_gather* functions below keep track of flushing the IOMMU TLB
> + * and unpinning the pages. It is safe to call them gather == NULL, in
> + * which case they will fall-back to flushing the TLB and unpinning the
> + * pages at every call.
> + */
> +static long vfio_gather_flush(struct iommu_domain *domain,
> + struct vfio_dma *dma,
> + struct vfio_gather *gather)
> +{
> + long unlocked = 0;
> + unsigned i;
> +
> + if (!gather)
|| !gather->fill
We might have gotten lucky that our last add triggered a flush.
> + goto out;
> +
> + /* First flush unmapped TLB entries */
> + iommu_tlb_sync(domain);
> +
> + for (i = 0; i < gather->fill; i++) {
> + dma_addr_t iova = gather->entries[i].iova;
> + phys_addr_t phys = gather->entries[i].phys;
> + size_t size = gather->entries[i].size;
> +
> + unlocked += vfio_unpin_pages_remote(dma, iova,
> + phys >> PAGE_SHIFT,
> + size >> PAGE_SHIFT,
> + false);
> + }
> +
> + gather->fill = 0;
A struct vfio_gather_entry* would clean this up and eliminate some
variables, including i.
> +
> +out:
> + return unlocked;
> +}
> +
> +static long vfio_gather_add(struct iommu_domain *domain,
> + struct vfio_dma *dma,
> + struct vfio_gather *gather,
> + dma_addr_t iova, phys_addr_t phys, size_t size)
> +{
> + long unlocked = 0;
> +
> + if (gather) {
> + unsigned index;
> +
> + if (gather->fill == GATHER_ENTRIES)
> + unlocked = vfio_gather_flush(domain, dma, gather);
unlocked += vfio_unpin_pages_remote(...);
} else {
IOW, vfio_gather_flush() has already done the iommu_tlb_sync() for the
mapping that called us, there's no point in adding these to our list,
unpin them immediate.
> +
> + index = gather->fill++;
> +
> + gather->entries[index].iova = iova;
> + gather->entries[index].phys = phys;
> + gather->entries[index].size = size;
Alternatively, do the test and flush here instead.
Thanks,
Alex
> + } else {
> + iommu_tlb_sync(domain);
> +
> + unlocked = vfio_unpin_pages_remote(dma, iova,
> + phys >> PAGE_SHIFT,
> + size >> PAGE_SHIFT,
> + false);
> + }
> +
> + return unlocked;
> +}
> +
> /*
> * This code handles mapping and unmapping of user data buffers
> * into DMA'ble space using the IOMMU
> @@ -653,6 +739,7 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> {
> dma_addr_t iova = dma->iova, end = dma->iova + dma->size;
> struct vfio_domain *domain, *d;
> + struct vfio_gather *gather;
> long unlocked = 0;
>
> if (!dma->size)
> @@ -662,6 +749,12 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> return 0;
>
> /*
> + * No need to check return value - It is safe to continue with a
> + * NULL pointer.
> + */
> + gather = kzalloc(sizeof(*gather), GFP_KERNEL);
> +
> + /*
> * We use the IOMMU to track the physical addresses, otherwise we'd
> * need a much more complicated tracking system. Unfortunately that
> * means we need to use one of the iommu domains to figure out the
> @@ -706,17 +799,20 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> break;
>
> iommu_tlb_range_add(domain->domain, iova, unmapped);
> - iommu_tlb_sync(domain->domain);
>
> - unlocked += vfio_unpin_pages_remote(dma, iova,
> - phys >> PAGE_SHIFT,
> - unmapped >> PAGE_SHIFT,
> - false);
> + unlocked += vfio_gather_add(domain->domain, dma, gather,
> + iova, phys, unmapped);
> +
> iova += unmapped;
>
> cond_resched();
> }
>
> + unlocked += vfio_gather_flush(domain->domain, dma, gather);
> +
> + kfree(gather);
> + gather = NULL;
> +
> dma->iommu_mapped = false;
> if (do_accounting) {
> vfio_lock_acct(dma->task, -unlocked, NULL);
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