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

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Message-ID: <a7781053-07ec-299a-0610-ece3c30bdb8e@redhat.com>
Date:   Fri, 1 Dec 2017 09:08:59 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maran Wilson <maran.wilson@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Roger Pau Monné <roger.pau@...rix.com>,
        Juergen Gross <jgross@...e.com>
Cc:     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 30/11/2017 19:23, Maran Wilson wrote:
> Are you saying the Linux PVH entry code (such as init_pvh_bootparams())
> should use the fw_cfg interface to read the e820 memory map data and put
> it into the zeropage? Basically, keeping the patch very much like it
> already is, just extracting the e820 data via the fw_cfg interface
> instead of from the second module of start_info struct?

Yes.

> If that is the case, I guess I'm a bit hesitant to throw the QEMU
> specific fw_cfg interface into the mix on the Linux PVH side when the
> existing PVH ABI already seems to contain an interface for passing
> modules/blobs to the guest. But if you feel there is a compelling reason
> to use the fw_cfg interface here, I'm happy to explore that approach
> further.

I think the same holds true for Xen, but it is still using a hypercall 
to get the memory map.  In the end, using fw_cfg seems closest to what 
the Xen code does.

There are other possibilities:

1) defining a v2 PVH ABI that includes the e820 map would also be a 
possibility.

2) modify enlighten_pvh.c to get the start info in multiboot format,
something like:

diff --git a/arch/x86/xen/enlighten_pvh.c b/arch/x86/xen/enlighten_pvh.c
index 98ab17673454..656e41449db0 100644
--- a/arch/x86/xen/enlighten_pvh.c
+++ b/arch/x86/xen/enlighten_pvh.c
@@ -88,19 +88,22 @@ void __init xen_prepare_pvh(void)
 	u32 msr;
 	u64 pfn;
 
-	if (pvh_start_info.magic != XEN_HVM_START_MAGIC_VALUE) {
+	if (pvh_start_info.magic == XEN_HVM_START_MAGIC_VALUE) {
+		xen_pvh = 1;
+
+		init_pvh_bootparams_xen();
+
+		msr = cpuid_ebx(xen_cpuid_base() + 2);
+		pfn = __pa(hypercall_page);
+		wrmsr_safe(msr, (u32)pfn, (u32)(pfn >> 32));
+
+		x86_init.oem.arch_setup = xen_pvh_arch_setup;
+	} else if (pvh_start_info.magic == MULTIBOOT_INFO_MAGIC_VALUE) {
+		init_pvh_bootparams_multiboot();
+
+	} else {
 		xen_raw_printk("Error: Unexpected magic value (0x%08x)\n",
 				pvh_start_info.magic);
 		BUG();
 	}
-
-	xen_pvh = 1;
-
-	msr = cpuid_ebx(xen_cpuid_base() + 2);
-	pfn = __pa(hypercall_page);
-	wrmsr_safe(msr, (u32)pfn, (u32)(pfn >> 32));
-
-	init_pvh_bootparams();
-
-	x86_init.oem.arch_setup = xen_pvh_arch_setup;
 }


Note that this would *not* be a multiboot-format kernel, as it would
still have the Xen PVH ELF note.  It would just reuse the format of
the start info struct.

However, I think it is simpler to just use the e820 memory map from
fw_cfg.

Paolo