linux-kernel - Re: [PATCH v3 0/4] KVM: allow mapping non-refcounted pages

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Date:   Tue, 28 Sep 2021 19:40:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Stevens <stevensd@...omium.org>,
        Marc Zyngier <maz@...nel.org>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 0/4] KVM: allow mapping non-refcounted pages

On 25/08/21 04:50, David Stevens wrote:
> From: David Stevens <stevensd@...omium.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(-)
>