Re: [RFC PATCH v4 0/7] KVM: x86: Allow Qemu/KVM to use PVH entry point

From: Paolo Bonzini <pbonzini@redhat.com>
To: Maran Wilson <maran.wilson@oracle.com>,
	jgross@suse.com, boris.ostrovsky@oracle.com,
	roger.pau@citrix.com, andrew.cooper3@citrix.com,
	hch@infradead.org, JBeulich@suse.com, x86@kernel.org,
	xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org
Cc: tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
	rkrcmar@redhat.com, jpoimboe@redhat.com, bp@suse.de,
	kirill.shutemov@linux.intel.com, thomas.lendacky@amd.com,
	luto@kernel.org, dave.hansen@linux.intel.com,
	davem@davemloft.net, gregkh@linuxfoundation.org,
	mchehab@kernel.org, linus.walleij@linaro.org,
	rdunlap@infradead.org
Subject: Re: [RFC PATCH v4 0/7] KVM: x86: Allow Qemu/KVM to use PVH entry point
Date: Wed, 28 Feb 2018 22:39:34 +0100	[thread overview]
Message-ID: <239b5f48-00ad-5ff3-aa4f-ba126596c808@redhat.com> (raw)
In-Reply-To: <1519842483-8887-1-git-send-email-maran.wilson@oracle.com>

On 28/02/2018 19:27, Maran Wilson wrote:
> Sorry for the delay between this version and the last -- it was mostly
> due to holidays and everyone being focused on security bug mitigation
> issues. Here are the links to the previous email threads in case it is
> helpful:
> 
> V3: https://lkml.org/lkml/2017/12/12/1230
> V2: https://lkml.org/lkml/2017/12/7/1624
> V1: https://lkml.org/lkml/2017/11/28/1280
> 
> Changes from v3:
> 
>  * Implemented Juergen's suggestion for refactoring and moving the PVH
>    code so that CONFIG_XEN is no longer required for booting KVM guests
>    via the PVH entry point.
>    Functionally, nothing has changed from V3 really, but the patches
>    look completely different now because of all the code movement and
>    refactoring. Some of these patches can be combined, but I've left
>    them very small in some cases to make the refactoring and code
>    movement easier to review.
>    My approach for refactoring has been to create a PVH entry layer that
>    still has understanding and knowledge about Xen vs non-Xen guest types
>    so that it can make run time decisions to handle either case, as
>    opposed to going all the way and re-writing it to be a completely
>    hypervisor agnostic and architecturally pure layer that is separate
>    from guest type details. The latter seemed a bit overkill in this
>    situation. And I've handled the complexity of having to support
>    Qemu/KVM boot of kernels compiled with or without CONFIG_XEN via a
>    pair of xen specific __weak routines that can be overridden in kernels
>    that support Xen guests. Importantly, the __weak routines are for
>    xen specific code only (not generic "guest type" specific code) so
>    there is no clashing between xen version of the strong routine and,
>    say, a KVM version of the same routine. But I'm sure there are many
>    ways to skin this cat, so I'm open to alternate suggestions if there
>    is a compelling reason for not using __weak in this situation.

As you say there are many ways to achieve this and I think your choice
is fully reasonable (the other alternative that comes to mind is a "Xen
detect" function that returns a struct of function pointers).

Apart from the placement of the files, it looks great.  Thanks!

Paolo