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Subject: Re: [Xen-devel] [PATCH RFC 00/39] x86/KVM: Xen HVM guest support
To:     =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= 
        <marmarek@invisiblethingslab.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        xen-devel@lists.xenproject.org,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        x86@kernel.org, Ankur Arora <ankur.a.arora@oracle.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>
References: <20190220201609.28290-1-joao.m.martins@oracle.com>
 <20190220233941.GA5279@mail-itl>
From:   Joao Martins <joao.m.martins@oracle.com>
Message-ID: <6c12ea67-0845-d2c2-6f78-f4819b46b7b5@oracle.com>
Date:   Thu, 21 Feb 2019 11:55:10 +0000
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On 2/20/19 11:39 PM, Marek Marczykowski-Górecki wrote:
> On Wed, Feb 20, 2019 at 08:15:30PM +0000, Joao Martins wrote:
>>  2. PV Driver support (patches 17 - 39)
>>
>>  We start by redirecting hypercalls from the backend to routines
>>  which emulate the behaviour that PV backends expect i.e. grant
>>  table and interdomain events. Next, we add support for late
>>  initialization of xenbus, followed by implementing
>>  frontend/backend communication mechanisms (i.e. grant tables and
>>  interdomain event channels). Finally, introduce xen-shim.ko,
>>  which will setup a limited Xen environment. This uses the added
>>  functionality of Xen specific shared memory (grant tables) and
>>  notifications (event channels).
> 
> Does it mean backends could be run in another guest, similarly as on
> real Xen? AFAIK virtio doesn't allow that as virtio backends need
> arbitrary write access to guest memory. But grant tables provide enough
> abstraction to do that safely.
> 
In this series not yet. Here we are trying to resemble how KVM drives its kernel
PV backends (i.e. vhost virtio).

The domU grant {un,}mapping could be added complementary. The main difference
between domU gntmap vs shim gntmap is that the former maps/unmaps the  guest
page on the backend. Most of it is common code I think.

	Joao