linux-kernel - Re: [RFC 00/12] KVM/X86: Introduce a new guest mapping API

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Date:   Mon, 05 Feb 2018 21:26:30 +0200
From:   Mihai Donțu <mdontu@...defender.com>
To:     KarimAllah Ahmed <karahmed@...zon.de>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC 00/12] KVM/X86: Introduce a new guest mapping API

On Mon, 2018-02-05 at 19:47 +0100, KarimAllah Ahmed wrote:
> Guest memory can either be directly managed by the kernel (i.e. have a "struct
> page") or they can simply live outside kernel control (i.e. do not have a
> "struct page"). KVM mostly support these two modes, except in a few places
> where the code seems to assume that guest memory must have a "struct page".

In cases where there is no 'struct page', would it be possible to get
two VM-s to share memory (in the way Xen's grant tables do)?

We are working on a page sharing mechanism and would like to know if
this use case can be accommodated.

Thank you,

> This patchset introduces a new mapping function to map guest memory into host
> kernel memory. Ideally I should also get rid of all guest mapping functions
> that end up converting a guest address to a page, but I decided to get feedback
> on this first and see if this is an acceptable API.
> 
> Most of the offending code paths that has been updated are in the nested code
> base. Mostly because I stumbled upon this code while looking at the nested MSR
> bitmap handling for the IBRS patches. There are also offending code paths in
> SVM code, but I will do that once the interface is accepted.
> 
> KarimAllah Ahmed (12):
>   KVM: Introduce helper functions to map/unmap guest memory
>   KVM/VMX: Use the new host mapping API for apic_access_page
>   KVM/VMX: Use the new host mapping API for virtual_apic_page
>   KVM/VMX: Use the new host mapping API for pi_desc_page
>   KVM/VMX: Use the new host mapping API for mapping nested vmptr
>   KVM/VMX: Use the new host mapping API for handle_vmptrld
>   KVM/VMX: Use the new host mapping API for mapping L12 MSR bitmap
>   KVM/VMX: Use the new host mapping API for mapping nested PML
>   KVM/VMX: Use the new host mapping API for cmpxchg_emulated
>   KVM/VMX: Use the new host mapping API for synic_clear_sint_msg_pending
>   KVM/VMX: Use the new host mapping API for synic_deliver_msg
>   KVM/VMX: Remove kvm_vcpu_gpa_to_page as it is now unused
> 
>  arch/x86/kvm/hyperv.c    |  24 +++----
>  arch/x86/kvm/vmx.c       | 159 ++++++++++++++++++++---------------------------
>  arch/x86/kvm/x86.c       |  12 ++--
>  include/linux/kvm_host.h |  18 +++++-
>  virt/kvm/kvm_main.c      |  62 ++++++++++++++++++
>  5 files changed, 161 insertions(+), 114 deletions(-)
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: kvm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org

-- 
Mihai Donțu