linux-kernel - Re: [PATCH RFC 2/5] KVM: x86: introduce KVM_MEM

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Date:   Thu, 14 May 2020 12:18:23 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michael Tsirkin <mst@...hat.com>,
        Julia Suvorova <jsuvorov@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org
Subject: Re: [PATCH RFC 2/5] KVM: x86: introduce KVM_MEM_ALLONES memory

On Thu, May 14, 2020 at 08:05:37PM +0200, Vitaly Kuznetsov wrote:
> PCIe config space can (depending on the configuration) be quite big but
> usually is sparsely populated. Guest may scan it by accessing individual
> device's page which, when device is missing, is supposed to have 'pci
> holes' semantics: reads return '0xff' and writes get discarded. Currently,
> userspace has to allocate real memory for these holes and fill them with
> '0xff'. Moreover, different VMs usually require different memory.
> 
> The idea behind the feature introduced by this patch is: let's have a
> single read-only page filled with '0xff' in KVM and map it to all such
> PCI holes in all VMs. This will free userspace of obligation to allocate
> real memory. Later, this will also allow us to speed up access to these
> holes as we can aggressively map the whole slot upon first fault.
> 
> Suggested-by: Michael S. Tsirkin <mst@...hat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  Documentation/virt/kvm/api.rst  | 22 ++++++---
>  arch/x86/include/uapi/asm/kvm.h |  1 +
>  arch/x86/kvm/x86.c              |  9 ++--
>  include/linux/kvm_host.h        | 15 ++++++-
>  include/uapi/linux/kvm.h        |  2 +
>  virt/kvm/kvm_main.c             | 79 +++++++++++++++++++++++++++++++--
>  6 files changed, 113 insertions(+), 15 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index d871dacb984e..2b87d588a7e0 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1236,7 +1236,8 @@ yet and must be cleared on entry.
>  
>    /* for kvm_memory_region::flags */
>    #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
> -  #define KVM_MEM_READONLY	(1UL << 1)
> +  #define KVM_MEM_READONLY		(1UL << 1)
> +  #define KVM_MEM_ALLONES		(1UL << 2)

Why not call this KVM_MEM_PCI_HOLE or something else that better conveys
that this is memslot is intended to emulate PCI master abort semantics?