Re: [PATCH] vhost: use large iotlb entry if no IOMMU translation is needed

[Chao Gao]

On Tue, Aug 03, 2021 at 12:43:58PM +0800, Jason Wang wrote:
>
>在 2021/8/3 下午12:29, Chao Gao 写道:
>> Ping. Could someone help to review this patch?
>> 
>> Thanks
>> Chao
>> 
>> On Wed, Jul 21, 2021 at 03:54:02PM +0800, Chao Gao wrote:
>> > If guest enables IOMMU_PLATFORM for virtio-net, severe network
>> > performance drop is observed even if there is no IOMMU.
>
>
>We see such reports internally and we're testing a patch series to disable
>vhost IOTLB in this case.
>
>Will post a patch soon.

OK. put me in the CC list. I would like to test with TDX to ensure your patch
fix the performance issue I am facing.

>
>
>
>> >   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.
>
>
>Instead of introducing new general facilities like unaligned IOTLB entry. I
>wonder if we optimize the vtd_iommu_translate() to use e.g 1G instead?

using 1G iotlb entry looks feasible.

>
>    } else {
>        /* DMAR disabled, passthrough, use 4k-page*/
>        iotlb.iova = addr & VTD_PAGE_MASK_4K;
>        iotlb.translated_addr = addr & VTD_PAGE_MASK_4K;
>        iotlb.addr_mask = ~VTD_PAGE_MASK_4K;
>        iotlb.perm = IOMMU_RW;
>        success = true;
>    }
>
>
>> >   Actually, vhost-net
>> > in kernel supports unaligned iotlb entry. The alignment requirement is
>> > imposed by address_space_get_iotlb_entry() and flatview_do_translate().
>
>
>For the passthrough case, is there anyway to detect them and then disable
>device IOTLB in those case?

yes. I guess so; qemu knows the presence and status of iommu. Currently,
in flatview_do_translate(), memory_region_get_iommu() tells whether a memory
region is behind an iommu.

Thanks
Chao