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Message-ID: <92a21507-b179-49ca-b0e0-d0aea69ab3a3@redhat.com>
Date: Fri, 23 Feb 2024 10:44:58 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: David Stevens <stevensd@...omium.org>,
Sean Christopherson <seanjc@...gle.com>
Cc: Yu Zhang <yu.c.zhang@...ux.intel.com>,
Isaku Yamahata <isaku.yamahata@...il.com>,
Zhi Wang <zhi.wang.linux@...il.com>, Maxim Levitsky <mlevitsk@...hat.com>,
kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v10 0/8] KVM: allow mapping non-refcounted pages
On 2/21/24 08:25, 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_follow_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.
>
> From there, this series extends the kvm_follow_pfn() API so that
> non-refconuted pages can be safely handled. This invloves adding an
> input parameter to indicate whether the caller can safely use
> non-refcounted pfns and an output parameter to tell the caller whether
> or not the returned page is refcounted. This change includes a breaking
> change, by disallowing non-refcounted pfn mappings by default, as such
> mappings are unsafe. To allow such systems to continue to function, an
> opt-in module parameter is added to allow the unsafe behavior.
>
> This series only adds support for non-refcounted pages to x86. Other
> MMUs can likely be updated without too much difficulty, but it is not
> needed at this point. Updating other parts of KVM (e.g. pfncache) is not
> straightforward [2].
Looks good to me, apart that two patches were sent twice. I only have a
small comment on patch 4, to which I'll reply separately.
Paolo
> [1]
> https://patchwork.kernel.org/project/dri-devel/cover/20200814024000.2485-1-gurchetansingh@chromium.org/
> [2] https://lore.kernel.org/all/ZBEEQtmtNPaEqU1i@google.com/
>
> v9 -> v10:
> - Re-add FOLL_GET changes.
> - Split x86/mmu spte+non-refcount-page patch into two patches.
> - Rename 'foll' variables to 'kfp'.
> - Properly gate usage of refcount spte bit when it's not available.
> - Replace kfm_follow_pfn's is_refcounted_page output parameter with
> a struct page *refcounted_page pointing to the page in question.
> - Add patch downgrading BUG_ON to WARN_ON_ONCE.
> v8 -> v9:
> - Make paying attention to is_refcounted_page mandatory. This means
> that FOLL_GET is no longer necessary. For compatibility with
> un-migrated callers, add a temporary parameter to sidestep
> ref-counting issues.
> - Add allow_unsafe_mappings, which is a breaking change.
> - Migrate kvm_vcpu_map and other callsites used by x86 to the new API.
> - Drop arm and ppc changes.
> v7 -> v8:
> - Set access bits before releasing mmu_lock.
> - Pass FOLL_GET on 32-bit x86 or !tdp_enabled.
> - Refactor FOLL_GET handling, add kvm_follow_refcounted_pfn helper.
> - Set refcounted bit on >4k pages.
> - Add comments and apply formatting suggestions.
> - rebase on kvm next branch.
> v6 -> v7:
> - Replace __gfn_to_pfn_memslot with a more flexible __kvm_faultin_pfn,
> and extend that API to support non-refcounted pages (complete
> rewrite).
>
> David Stevens (7):
> KVM: Relax BUG_ON argument validation
> KVM: mmu: Introduce kvm_follow_pfn()
> KVM: mmu: Improve handling of non-refcounted pfns
> KVM: Migrate kvm_vcpu_map() to kvm_follow_pfn()
> KVM: x86: Migrate to kvm_follow_pfn()
> KVM: x86/mmu: Track if sptes refer to refcounted pages
> KVM: x86/mmu: Handle non-refcounted pages
>
> Sean Christopherson (1):
> KVM: Assert that a page's refcount is elevated when marking
> accessed/dirty
>
> arch/x86/kvm/mmu/mmu.c | 104 +++++++---
> arch/x86/kvm/mmu/mmu_internal.h | 2 +
> arch/x86/kvm/mmu/paging_tmpl.h | 7 +-
> arch/x86/kvm/mmu/spte.c | 4 +-
> arch/x86/kvm/mmu/spte.h | 22 +-
> arch/x86/kvm/mmu/tdp_mmu.c | 22 +-
> arch/x86/kvm/x86.c | 11 +-
> include/linux/kvm_host.h | 53 ++++-
> virt/kvm/guest_memfd.c | 8 +-
> virt/kvm/kvm_main.c | 349 +++++++++++++++++++-------------
> virt/kvm/kvm_mm.h | 3 +-
> virt/kvm/pfncache.c | 11 +-
> 12 files changed, 399 insertions(+), 197 deletions(-)
>
>
> base-commit: 54be6c6c5ae8e0d93a6c4641cb7528eb0b6ba478
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