linux-kernel - Re: [PATCH v6 02/14] KVM: Cache as_id in kvm_memory

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Date:   Tue, 10 Mar 2020 07:48:59 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Yan Zhao <yan.y.zhao@...el.com>,
        Jason Wang <jasowang@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Christophe de Dinechin <dinechin@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v6 02/14] KVM: Cache as_id in kvm_memory_slot

On Mon, Mar 09, 2020 at 05:44:12PM -0400, Peter Xu wrote:
> Cache the address space ID just like the slot ID.  It will be used in
> order to fill in the dirty ring entries.
> 
> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  include/linux/kvm_host.h | 1 +
>  virt/kvm/kvm_main.c      | 2 ++
>  2 files changed, 3 insertions(+)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index bcb9b2ac0791..afa0e9034881 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -346,6 +346,7 @@ struct kvm_memory_slot {
>  	unsigned long userspace_addr;
>  	u32 flags;
>  	short id;
> +	u8 as_id;

My vote is to store this as a u16 and remove the BUILD_BUG_ON.  Using a u8
doesn't save any memory since the compiler will pad out the struct.

>  };
>  
>  static inline unsigned long kvm_dirty_bitmap_bytes(struct kvm_memory_slot *memslot)
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 70f03ce0e5c1..e6484dabfc59 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1036,6 +1036,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  
>  	new = old = *slot;
>  
> +	BUILD_BUG_ON(U8_MAX < KVM_ADDRESS_SPACE_NUM);
> +	new.as_id = as_id;
>  	new.id = id;
>  	new.base_gfn = base_gfn;
>  	new.npages = npages;
> -- 
> 2.24.1
>