linux-kernel - Re: [RFC PATCH v3 4/8] vfio/type1: Pre-map more pages than requested in the IOPF handling

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Message-ID: <20210518125826.7ba38b78.alex.williamson@redhat.com>
Date:   Tue, 18 May 2021 12:58:26 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Shenming Lu <lushenming@...wei.com>
Cc:     Cornelia Huck <cohuck@...hat.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Eric Auger <eric.auger@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>, <linux-api@...r.kernel.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>, <yi.l.liu@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        <wanghaibin.wang@...wei.com>, <yuzenghui@...wei.com>
Subject: Re: [RFC PATCH v3 4/8] vfio/type1: Pre-map more pages than
 requested in the IOPF handling

On Fri, 9 Apr 2021 11:44:16 +0800
Shenming Lu <lushenming@...wei.com> wrote:

> To optimize for fewer page fault handlings, we can pre-map more pages
> than requested at once.
> 
> Note that IOPF_PREMAP_LEN is just an arbitrary value for now, which we
> could try further tuning.

I'd prefer that the series introduced full end-to-end functionality
before trying to improve performance.  The pre-map value seems
arbitrary here and as noted in the previous patch, the IOMMU API does
not guarantee unmaps of ranges smaller than the original mapping.  This
would need to map with single page granularity in order to guarantee
page granularity at the mmu notifier when the IOMMU supports
superpages.  Thanks,

Alex