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

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Juergen Gross" <jgross@suse.com>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Cc: Maran Wilson <maran.wilson@oracle.com>,
	tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
	x86@kernel.org, xen-devel@lists.xenproject.org,
	linux-kernel@vger.kernel.org, rkrcmar@redhat.com,
	JBeulich@suse.com, andrew.cooper3@citrix.com,
	kvm@vger.kernel.org
Subject: Re: [RFC PATCH] KVM: x86: Allow Qemu/KVM to use PVH entry point
Date: Wed, 29 Nov 2017 15:50:27 +0100	[thread overview]
Message-ID: <1a529a09-2b84-a730-7518-158489303377@redhat.com> (raw)
In-Reply-To: <e46a8232-22c0-e437-6bd8-e51844a60eaa@suse.com>

On 29/11/2017 15:47, Juergen Gross wrote:
> On 29/11/17 15:44, Paolo Bonzini wrote:
>> In either case, you would have a new option ROM.  It could either be
>> very simple and similar to multiboot.S, or it could be larger and do the
>> same task as xen-pvh.S and enlighten_pvh.c (then get the address of
>> startup_32 or startup_64 from FW_CFG_KERNEL_ENTRY and jump there).  The
>> ugly part is that the option ROM would have to know more details about
>> what it is going to boot, including for example whether it's 32-bit or
>> 64-bit, so I don't really think it is a good idea.
> 
> As grub2 doesn't have to know, qemu shouldn't have to know either.

That would be exactly what linuxboot-dma.c does already, but it's slower
than PVH because Linux has to uncompress itself.

The above thought experiment would make QEMU able to boot a PVH kernel
without any changes to Linux.

Paolo