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From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Subject: Re: Communication in x86
Date: Tue, 28 Mar 2017 16:51:41 +0200
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Cc: kvm@vger.kernel.org, Thomas Huth <thuth@redhat.com>
To: James Edouard <jedouard1994@gmail.com>
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Disclaimer: I have very little knowledge about PPC.

2017-03-16 11:43-0400, James Edouard:
> Hi,
> 
> The hypercall KVM_HC_PPC_MAP_MAGIC_PAGE is only implemented for PPC,
> but I am wondering why there is not a similar hypercall for x86?

PPC traps all privileged instructions while emulating privileged guest
mode and the magic page accelerates instructions where the trap would be
trivial.  By patching instructions to access the page instead.

x86 allows execution of most performance sensitive privileged
instructions without a trap from guest mode.

>                                                                  Is it
> not needed (I'm a complete newbie...)?

Yes, it is optional at best.  I'm not aware of any x86 instruction where
adding a magic page for its emulation would be a clearly good idea.

While there is no magic page in the PPC sense on x86, there are areas of
guest memory where the host writes some data for the guest to read
without a trap, but they are also usually setup using MSR interfaces:
kvm clock, steal time, ...