Support for XSAVES intel instructions in QEMU

[Alexandre Bécholey]

Dear QEMU developers,

I am running Hyper-V on qemu+kvm. During it initialization, it checks
for XSAVES support: first it executes CPUID with EAX = 0xd and ECX = 1
and looks at bit 3 in the returned value of EAX (Supports
XSAVES/XRSTORS and IA32_XSS [1]), and then it reads the MSR
IA32_VMX_PROCBASED_CTLS2 (index 0x48B) and looks at bit 20 (Enable
XSAVES/XSTORS [2]). If there is any inconsistencies between the two,
i.e. CPUID shows that XSAVES is supported and the bit is not enabled
in the MSR, Hyper-V decides to fail and stops its initialization. It
used to work until last spring/summer where something might have
changed in either KVM or QEMU.

It seems that KVM sets the correct flags (in CPUID and the MSR) when
the host CPU supports XSAVES. In QEMU, based on comments in
target/i386/cpu.c it seems that XSAVES is not added in
builtin_x86_defs[].features[FEAT_VMX_SECONDARY_CTLS] because it might
break live migration. Therefore, when setting the MSR for the vcpu,
QEMU is masking off the feature.

I have tested two possible solutions:
- adding the flag in .features[FEAT_VMX_SECONDARY_CTLS]
- removing the support of the instruction in
feature_word_info[FEAT_XSAVE].feat_names

Both solutions work and Hyper-v is happily running. I can provide a
patch for the solution you might consider applying. Otherwise, is
there a better way to fix the issue?

Many thanks,
Alexandre

Ref:
[1] Intel SDM Volume 2A, chapter 3, page 196
[2] Intel SDM Volume 3C, chapter 24, page 11