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SubjectRe: [PATCH v3 0/4] KVM: allow mapping non-refcounted pages
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On 25/08/21 04:50, David Stevens wrote:
> From: David Stevens <stevensd@chromium.org>
>
> This patch series adds support for mapping non-refcount VM_IO and
> VM_PFNMAP memory into the guest.
>
> Currently, the gfn_to_pfn functions require being able to pin the target
> pfn, so they will fail if the pfn returned by follow_pte isn't a
> ref-counted page. However, the KVM secondary MMUs do not require that
> the pfn be pinned, since they are integrated with the mmu notifier API.
> This series adds a new set of gfn_to_pfn_page functions which parallel
> the gfn_to_pfn functions but do not pin the pfn. The new functions
> return the page from gup if it was present, so callers can use it and
> call put_page when done.
>
> The gfn_to_pfn functions should be depreciated, since as they are unsafe
> due to relying on trying to obtain a struct page from a pfn returned by
> follow_pte. I added new functions instead of simply adding another
> optional parameter to the existing functions to make it easier to track
> down users of the deprecated functions.
>
> This series updates x86 and arm64 secondary MMUs to the new API.
>
> v2 -> v3:
> - rebase on kvm next branch

Hi David,

this needs a rebase. I have pushed my current queue, but note that
parts of it are still untested.

A bigger question here is the gfn_to_pfn caches and how to properly
invalidate them. However your patch doesn't make things worse (only a
bit inconsistent because pointing certain MSRs to a VM_PFNMAP|VM_IO page
can fail).

Paolo

> v1 -> v2:
> - Introduce new gfn_to_pfn_page functions instead of modifying the
> behavior of existing gfn_to_pfn functions, to make the change less
> invasive.
> - Drop changes to mmu_audit.c
> - Include Nicholas Piggin's patch to avoid corrupting refcount in the
> follow_pte case, and use it in depreciated gfn_to_pfn functions.
> - Rebase on kvm/next
>
> David Stevens (4):
> KVM: mmu: introduce new gfn_to_pfn_page functions
> KVM: x86/mmu: use gfn_to_pfn_page
> KVM: arm64/mmu: use gfn_to_pfn_page
> KVM: mmu: remove over-aggressive warnings
>
> arch/arm64/kvm/mmu.c | 26 +++--
> arch/x86/kvm/mmu/mmu.c | 50 +++++----
> arch/x86/kvm/mmu/mmu_internal.h | 3 +-
> arch/x86/kvm/mmu/paging_tmpl.h | 23 ++--
> arch/x86/kvm/mmu/tdp_mmu.c | 6 +-
> arch/x86/kvm/mmu/tdp_mmu.h | 4 +-
> arch/x86/kvm/x86.c | 6 +-
> include/linux/kvm_host.h | 17 +++
> virt/kvm/kvm_main.c | 188 +++++++++++++++++++++++---------
> 9 files changed, 220 insertions(+), 103 deletions(-)
>

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