Re: [Qemu-devel] [Help] Windows2012 as Guest 64+cores on KVM Halts

[Gonglei (Arei)]

Hi,

> 
> On Sat, 2017-02-11 at 10:39 -0500, Paolo Bonzini wrote:
> > >
> > >
> > > >
> > > > On 10/02/2017 10:31, Gonglei (Arei) wrote:
> > > > >
> > > > > But We tested the same cases on Xen platform and VMware, and
> > > > > the guest booted successfully.
> > > >
> > > > Were these two also tested with enlightenments enabled?  TCG
> > > > surely isn't.
> > >
> > > About TCG, I just remove ' accel=kvm,' and 'hy_releaxed' from the
> > > below QEMU
> > > Command line, I thought the hyper-V enabled then. Sorry about that.
> > >
> > > But for Xen, we set 'viridian=1' which be thought the Hyper-V is
> > > enabled.
> > >
> > > For VMWare we also enabled the Hyper-V enlightenments.
> If I'm not mistaken, even Hyper-V server doesn't allow specify more
> than 64 vCPUs for Generation 1 VMs.

Normally yes, but I found the explanation from Microsoft document about it:

Maximum Supported Virtual Processors

On Windows operating systems versions through Windows Server 2008 R2, 
reporting the HV#1 hypervisor interface limits the Windows virtual machine 
to a maximum of 64 VPs, regardless of what is reported via CPUID.40000005.EAX.
Starting with Windows Server 2012 and Windows 8, if CPUID.40000005.EAX 
contains a value of -1, Windows assumes that the hypervisor imposes no specific
limit to the number of VPs. In this case, Windows Server 2012 guest VMs may
use more than 64 VPs, up to the maximum supported number of processors 
applicable to the specific Windows version being used.

Link: 
https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs

"Requirements for Implementing the Microsoft Hypervisor Interface"

And the below patch works for me, I can support max 255 vcpus for WS2012
with hyper-v enlightenments.

diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index 27fd050..efe3cbc 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -772,7 +772,7 @@ int kvm_arch_init_vcpu(CPUState *cs)

         c = &cpuid_data.entries[cpuid_i++];
         c->function = HYPERV_CPUID_IMPLEMENT_LIMITS;
-        c->eax = 0x40;
+        c->eax = -1;
         c->ebx = 0x40;

         kvm_base = KVM_CPUID_SIGNATURE_NEXT;

> In any case, if you are only interested in hv_relaxed, you can drop it
> off for WS2012 as long as you have cpu hypervisor flag
> (CPUID.1:ECX [bit 31]=1) turned on.
> 
hy_relaxed is just a example of enabling hyperv-v enlightenments.

Thanks,
-Gonglei