linux-kernel - Re: [PATCH v9 0/6] KVM: allow mapping non-refcounted pages

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Date:   Thu, 28 Sep 2023 22:19:36 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     David Stevens <stevensd@...omium.org>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        Zhi Wang <zhi.wang.linux@...il.com>, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v9 0/6] KVM: allow mapping non-refcounted pages

On Mon, Sep 11, 2023 at 11:16:30AM +0900, David Stevens wrote:
> From: David Stevens <stevensd@...omium.org>
> 
> This patch series adds support for mapping VM_IO and VM_PFNMAP memory
> that is backed by struct pages that aren't currently being refcounted
> (e.g. tail pages of non-compound higher order allocations) into the
> guest.
> 
> Our use case is virtio-gpu blob resources [1], which directly map host
> graphics buffers into the guest as "vram" for the virtio-gpu device.
> This feature currently does not work on systems using the amdgpu driver,
> as that driver allocates non-compound higher order pages via
> ttm_pool_alloc_page.
> 
> First, this series replaces the __gfn_to_pfn_memslot API with a more
> extensible __kvm_faultin_pfn API. The updated API rearranges
> __gfn_to_pfn_memslot's args into a struct and where possible packs the
> bool arguments into a FOLL_ flags argument. The refactoring changes do
> not change any behavior.

Instead of adding hacks to kvm you really should fix the driver / TTM
to not do weird memory allocations.