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Re: [Xen-devel] [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?


> 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 

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",
-       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


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