netdev - AMD IOMMU problem after NIC uses multi-page allocation

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Message-ID: <20230329181407.3eed7378@kernel.org>
Date:   Wed, 29 Mar 2023 18:14:07 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Joerg Roedel <joro@...tes.org>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc:     iommu@...ts.linux.dev,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Saeed Mahameed <saeed@...nel.org>
Subject: AMD IOMMU problem after NIC uses multi-page allocation

Hi Joerg, Suravee,

I see an odd NIC behavior with AMD IOMMU in lazy mode (on 5.19).

The NIC allocates a buffer for Rx packets which is MTU rounded up 
to page size. If I run it with 1500B MTU or 9000 MTU everything is
fine, slight but manageable perf hit.

But if I flip the MTU to 9k, run some traffic and then go back to 1.5k 
- 70%+ of CPU cycles are spent in alloc_iova (and children).

Does this ring any bells?