If guest enables IOMMU_PLATFORM for virtio-net, severe network performance drop is observed even if there is no IOMMU. And disabling vhost can mitigate the perf issue. Finally, we found the culprit is frequent iotlb misses: kernel vhost-net has 2048 entries and each entry is 4K (qemu uses 4K for i386 if no IOMMU); vhost-net can cache translations for up to 8M (i.e. 4K*2048) IOVAs. If guest uses >8M memory for DMA, there are some iotlb misses.
If there is no IOMMU or IOMMU is disabled or IOMMU works in pass-thru mode, we can optimistically use large, unaligned iotlb entries instead of 4K-aligned entries to reduce iotlb pressure. Actually, vhost-net in kernel supports unaligned iotlb entry. The alignment requirement is imposed by address_space_get_iotlb_entry() and flatview_do_translate(). Introduce IOMMUTLBEntryUnaligned which has a @len field to specify the iotlb size to abstract a generic iotlb entry: aligned (original IOMMUTLBEntry) and unaligned entry. flatview_do_translate() now returns a magic value in @page_mask_out if no IOMMU translation is needed. Then, address_space_get_iotbl_entry() can accordingly return a page-aligned iotlb entry or the whole memory region section where the iova resides as a large iotlb entry. Signed-off-by: Chao Gao <chao....@intel.com> --- hw/virtio/vhost.c | 6 +++--- include/exec/memory.h | 16 ++++++++++++++-- softmmu/physmem.c | 37 +++++++++++++++++++++++++++++-------- 3 files changed, 46 insertions(+), 13 deletions(-) diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c index e8f85a5d2d..6745caa129 100644 --- a/hw/virtio/vhost.c +++ b/hw/virtio/vhost.c @@ -1010,7 +1010,7 @@ static int vhost_memory_region_lookup(struct vhost_dev *hdev, int vhost_device_iotlb_miss(struct vhost_dev *dev, uint64_t iova, int write) { - IOMMUTLBEntry iotlb; + IOMMUTLBEntryUnaligned iotlb; uint64_t uaddr, len; int ret = -EFAULT; @@ -1031,8 +1031,8 @@ int vhost_device_iotlb_miss(struct vhost_dev *dev, uint64_t iova, int write) goto out; } - len = MIN(iotlb.addr_mask + 1, len); - iova = iova & ~iotlb.addr_mask; + len = MIN(iotlb.len, len); + iova = iotlb.iova; ret = vhost_backend_update_device_iotlb(dev, iova, uaddr, len, iotlb.perm); diff --git a/include/exec/memory.h b/include/exec/memory.h index c3d417d317..3f04e8fe88 100644 --- a/include/exec/memory.h +++ b/include/exec/memory.h @@ -94,6 +94,7 @@ struct MemoryRegionSection { }; typedef struct IOMMUTLBEntry IOMMUTLBEntry; +typedef struct IOMMUTLBEntryUnaligned IOMMUTLBEntryUnaligned; /* See address_space_translate: bit 0 is read, bit 1 is write. */ typedef enum { @@ -113,6 +114,15 @@ struct IOMMUTLBEntry { IOMMUAccessFlags perm; }; +/* IOMMUTLBEntryUnaligned may be not page-aligned */ +struct IOMMUTLBEntryUnaligned { + AddressSpace *target_as; + hwaddr iova; + hwaddr translated_addr; + hwaddr len; + IOMMUAccessFlags perm; +}; + /* * Bitmap for different IOMMUNotifier capabilities. Each notifier can * register with one or multiple IOMMU Notifier capability bit(s). @@ -2653,8 +2663,10 @@ void address_space_cache_destroy(MemoryRegionCache *cache); /* address_space_get_iotlb_entry: translate an address into an IOTLB * entry. Should be called from an RCU critical section. */ -IOMMUTLBEntry address_space_get_iotlb_entry(AddressSpace *as, hwaddr addr, - bool is_write, MemTxAttrs attrs); +IOMMUTLBEntryUnaligned address_space_get_iotlb_entry(AddressSpace *as, + hwaddr addr, + bool is_write, + MemTxAttrs attrs); /* address_space_translate: translate an address range into an address space * into a MemoryRegion and an address range into that section. Should be diff --git a/softmmu/physmem.c b/softmmu/physmem.c index 3c1912a1a0..469963f754 100644 --- a/softmmu/physmem.c +++ b/softmmu/physmem.c @@ -143,6 +143,8 @@ typedef struct subpage_t { #define PHYS_SECTION_UNASSIGNED 0 +#define PAGE_MASK_NOT_BEHIND_IOMMU ((hwaddr)-1) + static void io_mem_init(void); static void memory_map_init(void); static void tcg_log_global_after_sync(MemoryListener *listener); @@ -470,7 +472,9 @@ unassigned: * @page_mask_out: page mask for the translated address. This * should only be meaningful for IOMMU translated * addresses, since there may be huge pages that this bit - * would tell. It can be @NULL if we don't care about it. + * would tell. If the returned memory region section isn't + * behind an IOMMU, PAGE_MASK_NOT_BEHIND_IOMMU is return. + * It can be @NULL if we don't care about it. * @is_write: whether the translation operation is for write * @is_mmio: whether this can be MMIO, set true if it can * @target_as: the address space targeted by the IOMMU @@ -508,16 +512,18 @@ static MemoryRegionSection flatview_do_translate(FlatView *fv, target_as, attrs); } if (page_mask_out) { - /* Not behind an IOMMU, use default page size. */ - *page_mask_out = ~TARGET_PAGE_MASK; + /* return a magic value if not behind an IOMMU */ + *page_mask_out = PAGE_MASK_NOT_BEHIND_IOMMU; } return *section; } /* Called from RCU critical section */ -IOMMUTLBEntry address_space_get_iotlb_entry(AddressSpace *as, hwaddr addr, - bool is_write, MemTxAttrs attrs) +IOMMUTLBEntryUnaligned address_space_get_iotlb_entry(AddressSpace *as, + hwaddr addr, + bool is_write, + MemTxAttrs attrs) { MemoryRegionSection section; hwaddr xlat, page_mask; @@ -535,21 +541,36 @@ IOMMUTLBEntry address_space_get_iotlb_entry(AddressSpace *as, hwaddr addr, goto iotlb_fail; } + /* + * If the section isn't behind an IOMMU, return the whole section as an + * IOMMU TLB entry. + */ + if (page_mask == PAGE_MASK_NOT_BEHIND_IOMMU) { + return (IOMMUTLBEntryUnaligned) { + .target_as = as, + .iova = section.offset_within_address_space, + .translated_addr = section.offset_within_address_space, + .len = section.size, + /* IOTLBs are for DMAs, and DMA only allows on RAMs. */ + .perm = IOMMU_RW, + }; + } + /* Convert memory region offset into address space offset */ xlat += section.offset_within_address_space - section.offset_within_region; - return (IOMMUTLBEntry) { + return (IOMMUTLBEntryUnaligned) { .target_as = as, .iova = addr & ~page_mask, .translated_addr = xlat & ~page_mask, - .addr_mask = page_mask, + .len = page_mask + 1, /* IOTLBs are for DMAs, and DMA only allows on RAMs. */ .perm = IOMMU_RW, }; iotlb_fail: - return (IOMMUTLBEntry) {0}; + return (IOMMUTLBEntryUnaligned) {0}; } /* Called from RCU critical section */ -- 2.25.1