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

From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>,
	"Juergen Gross" <jgross@suse.com>
Cc: rkrcmar@redhat.com, kvm@vger.kernel.org, pbonzini@redhat.com,
	Maran Wilson <maran.wilson@oracle.com>,
	andrew.cooper3@citrix.com, x86@kernel.org,
	linux-kernel@vger.kernel.org, mingo@redhat.com,
	JBeulich@suse.com, hpa@zytor.com, xen-devel@lists.xenproject.org,
	tglx@linutronix.de
Subject: Re: [RFC PATCH] KVM: x86: Allow Qemu/KVM to use PVH entry point
Date: Wed, 29 Nov 2017 09:03:27 -0500	[thread overview]
Message-ID: <4d213199-ea65-4410-5b7a-63038215e380__46124.4813436226$1511964321$gmane$org@oracle.com> (raw)
In-Reply-To: <20171129085044.kc3yqqdcw3zmp2k2@MacBook-Pro-de-Roger.local>

On 11/29/2017 03:50 AM, Roger Pau Monné wrote:
> On Wed, Nov 29, 2017 at 09:21:59AM +0100, Juergen Gross wrote:
>> On 28/11/17 20:34, Maran Wilson wrote:
>>> For certain applications it is desirable to rapidly boot a KVM virtual
>>> machine. In cases where legacy hardware and software support within the
>>> guest is not needed, Qemu should be able to boot directly into the
>>> uncompressed Linux kernel binary without the need to run firmware.
>>>
>>> There already exists an ABI to allow this for Xen PVH guests and the ABI is
>>> supported by Linux and FreeBSD:
>>>
>>>    https://xenbits.xen.org/docs/unstable/misc/hvmlite.html
> I would also add a link to:
>
> http://xenbits.xen.org/docs/unstable/hypercall/x86_64/include,public,arch-x86,hvm,start_info.h.html#Struct_hvm_start_info
>
>>> This PoC patch enables Qemu to use that same entry point for booting KVM
>>> guests.
>>>
>>> Even though the code is still PoC quality, I'm sending this as an RFC now
>>> since there are a number of different ways the specific implementation
>>> details can be handled. I chose a shared code path for Xen and KVM guests
>>> but could just as easily create a separate code path that is advertised by
>>> a different ELF note for KVM. There also seems to be some flexibility in
>>> how the e820 table data is passed and how (or if) it should be identified
>>> as e820 data. As a starting point, I've chosen the options that seem to
>>> result in the smallest patch with minimal to no changes required of the
>>> x86/HVM direct boot ABI.
>> I like the idea.
>>
>> I'd rather split up the different hypervisor types early and use a
>> common set of service functions instead of special casing xen_guest
>> everywhere. This would make it much easier to support the KVM PVH
>> boot without the need to configure the kernel with CONFIG_XEN.
>>
>> Another option would be to use the same boot path as with grub: set
>> the boot params in zeropage and start at startup_32.
> I think I prefer this approach since AFAICT it should allow for
> greater code share with the common boot path.

zeropage is x86/Linux-specific so we'd need some sort of firmware (like
grub) between a hypervisor and Linux to convert hvm_start_info to
bootparams.

-boris

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