linux-kernel - Re: [PATCH 2/6] KVM: mmu: also return page from gfn_to

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Date:   Thu, 24 Jun 2021 10:40:00 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     David Stevens <stevensd@...omium.org>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        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>,
        Vitaly Kuznetsov <vkuznets@...hat.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, linux-mips@...r.kernel.org,
        kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/6] KVM: mmu: also return page from gfn_to_pfn

Hi David,

On Thu, 24 Jun 2021 04:57:45 +0100,
David Stevens <stevensd@...omium.org> wrote:
> 
> From: David Stevens <stevensd@...omium.org>
> 
> Return a struct kvm_pfn_page containing both a pfn and an optional
> struct page from the gfn_to_pfn family of functions. This differentiates
> the gup and follow_fault_pfn cases, which allows callers that only need
> a pfn to avoid touching the page struct in the latter case. For callers
> that need a struct page, introduce a helper function that unwraps a
> struct kvm_pfn_page into a struct page. This helper makes the call to
> kvm_get_pfn which had previously been in hva_to_pfn_remapped.
> 
> For now, wrap all calls to gfn_to_pfn functions in the new helper
> function. Callers which don't need the page struct will be updated in
> follow-up patches.
> 
> Signed-off-by: David Stevens <stevensd@...omium.org>
> ---
>  arch/arm64/kvm/mmu.c                   |   5 +-
>  arch/mips/kvm/mmu.c                    |   3 +-
>  arch/powerpc/kvm/book3s.c              |   3 +-
>  arch/powerpc/kvm/book3s_64_mmu_hv.c    |   5 +-
>  arch/powerpc/kvm/book3s_64_mmu_radix.c |   5 +-
>  arch/powerpc/kvm/book3s_hv_uvmem.c     |   4 +-
>  arch/powerpc/kvm/e500_mmu_host.c       |   2 +-
>  arch/x86/kvm/mmu/mmu.c                 |  11 ++-
>  arch/x86/kvm/mmu/mmu_audit.c           |   2 +-
>  arch/x86/kvm/x86.c                     |   2 +-
>  drivers/gpu/drm/i915/gvt/kvmgt.c       |   2 +-
>  include/linux/kvm_host.h               |  27 ++++--
>  include/linux/kvm_types.h              |   5 +
>  virt/kvm/kvm_main.c                    | 121 +++++++++++++------------
>  14 files changed, 109 insertions(+), 88 deletions(-)
> 

[...]

> +kvm_pfn_t kvm_pfn_page_unwrap(struct kvm_pfn_page pfnpg)
> +{
> +	if (pfnpg.page)
> +		return pfnpg.pfn;
> +
> +	kvm_get_pfn(pfnpg.pfn);
> +	return pfnpg.pfn;
> +}
> +EXPORT_SYMBOL_GPL(kvm_pfn_page_unwrap);

I'd really like to see a tiny bit of documentation explaining that
calls to kvm_pfn_page_unwrap() are not idempotent.

Otherwise, looks good to me.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.