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

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Message-ID: <1a529a09-2b84-a730-7518-158489303377@redhat.com>
Date:   Wed, 29 Nov 2017 15:50:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Roger Pau Monné <roger.pau@...rix.com>
Cc:     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,
        kvm@...r.kernel.org
Subject: Re: [RFC PATCH] KVM: x86: Allow Qemu/KVM to use PVH entry point

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