On 2014/12/18 6:17, Stefan Fritsch wrote:
On Thursday 11 December 2014 18:15:39, Paolo Bonzini wrote:
On 11/12/2014 13:53, zhanghailiang wrote:
I think it's a Windows bug---it should prefer x2apic to hv-vapic
if both are available.
No, i don't think it is a windows bug, it has nothing to do with
x2apic,
hv-vapic MSRs doesn't provide any performance improvement over
x2apic MSRs. So Windows should use x2apic MSRs if both are
available. Windows can use x2apic MSRs together with its EOI
optimization, like Linux does.
There definitely are Windows versions that know how to use x2apic
(e.g. 2008R2).
The Hyper-V specification (V3) has different bits for "supported" and
"recommended".
qemu always sets this bit (HV_X64_APIC_ACCESS_RECOMMENDED):
Bit 3: Recommend using MSRs for accessing APIC registers
EOI, ICR and TPR rather than their memory-mapped
counterparts.
But it does not seem to set this bit:
Bit 8: Recommend using x2APIC MSRs.
I guess that is why Windows does not use x2apic and qemu should always
I have test it when set bit 8, it seems that it is useless. The result
is no difference, compared with not setting this bit.
set this bit. But the HV_X64_APIC_ACCESS_RECOMMENDED bit should
probably not be set if the host supports apic-v. I havn't done any
That is what this patch want to do ;)
testing, though.
but apic-v (need hardware support, i.e. Haswell cpu).
APICv can use the x2apic MSRs.
When we don't passthough host cpu model to Guest os,
it has no idea about whether it supports apic-v in host,
The presence of APICv should be totally transparent to the guest.
Actually, qemu has a option 'hv_vapic' for -cpu, we can choose not
to configure it if we know there is apic-v support in host. But
IMHO, we'd better to do it automatically.
... and cause the CPUID to change under the guest's feet if you
migrate from a non-APICv to an APICv machines, or vice versa.
.
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