linux-kernel - Re: [PATCH v2 01/43] KVM: Prepare for handling only shared mappings in mmu

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Date: Thu, 25 Apr 2024 10:48:50 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Steven Price <steven.price@....com>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev, 
	Sean Christopherson <seanjc@...gle.com>, Catalin Marinas <catalin.marinas@....com>, 
	Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>, James Morse <james.morse@....com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Zenghui Yu <yuzenghui@...wei.com>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Joey Gouly <joey.gouly@....com>, 
	Alexandru Elisei <alexandru.elisei@....com>, Christoffer Dall <christoffer.dall@....com>, 
	linux-coco@...ts.linux.dev, 
	Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Subject: Re: [PATCH v2 01/43] KVM: Prepare for handling only shared mappings
 in mmu_notifier events

Hi,

On Fri, Apr 12, 2024 at 9:43 AM Steven Price <steven.price@....com> wrote:
>
> From: Sean Christopherson <seanjc@...gle.com>
>
> Add flags to "struct kvm_gfn_range" to let notifier events target only
> shared and only private mappings, and write up the existing mmu_notifier
> events to be shared-only (private memory is never associated with a
> userspace virtual address, i.e. can't be reached via mmu_notifiers).
>
> Add two flags so that KVM can handle the three possibilities (shared,
> private, and shared+private) without needing something like a tri-state
> enum.
>
> Link: https://lore.kernel.org/all/ZJX0hk+KpQP0KUyB@google.com
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Steven Price <steven.price@....com>
> ---
>  include/linux/kvm_host.h | 2 ++
>  virt/kvm/kvm_main.c      | 7 +++++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 48f31dcd318a..c7581360fd88 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -268,6 +268,8 @@ struct kvm_gfn_range {
>         gfn_t start;
>         gfn_t end;
>         union kvm_mmu_notifier_arg arg;
> +       bool only_private;
> +       bool only_shared;
>         bool may_block;
>  };
>  bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index fb49c2a60200..3486ceef6f4e 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -633,6 +633,13 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
>                          * the second or later invocation of the handler).
>                          */
>                         gfn_range.arg = range->arg;
> +
> +                       /*
> +                        * HVA-based notifications aren't relevant to private
> +                        * mappings as they don't have a userspace mapping.
> +                        */
> +                       gfn_range.only_private = false;
> +                       gfn_range.only_shared = true;
>                         gfn_range.may_block = range->may_block;

I'd discussed this with Sean when he posted this earlier. Having two
booleans to encode three valid states could be confusing. In response,
Sean suggested using an enum instead:
https://lore.kernel.org/all/ZUO1Giju0GkUdF0o@google.com/

Cheers,
/fuad

>
>                         /*


> --
> 2.34.1
>