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

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Message-ID: <20171129141810.q3s3xflsflpjovdd@MacBook-Pro-de-Roger.local>
Date:   Wed, 29 Nov 2017 14:18:10 +0000
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Juergen Gross <jgross@...e.com>
CC:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Maran Wilson <maran.wilson@...cle.com>, <tglx@...utronix.de>,
        <mingo@...hat.com>, <hpa@...or.com>, <x86@...nel.org>,
        <xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
        <rkrcmar@...hat.com>, <JBeulich@...e.com>,
        <andrew.cooper3@...rix.com>, <pbonzini@...hat.com>,
        <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH] KVM: x86: Allow Qemu/KVM to use PVH entry point

On Wed, Nov 29, 2017 at 03:11:12PM +0100, Juergen Gross wrote:
> On 29/11/17 15:03, Boris Ostrovsky wrote:
> > 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.
> 
> qemu?

But then it won't be using the PVH entry point, and would just use the
native one?

My understanding was that the PVH shim inside of Linux will prepare a
zero-page when booted using the PVH entry point, and then jump into
the native boot path.

Roger.