On Fri, Sep 21, 2018 at 11:22:10AM +0300, Roman Kagan wrote:
> Certain configurations do not allow SynIC to be used in QEMU. In
> particular,
>
> - when hyperv_vpindex is off, SINT routes can't be used as they refer to
> the destination vCPU by vp_index
>
> - older KVM (which doesn't expose KVM_CAP_HYPERV_SYNIC2) zeroes out
> SynIC message and event pages on every msr load, breaking migration
>
> OTOH in-KVM users of SynIC -- SynIC timers -- do work in those
> configurations, and we shouldn't stop the guest from using them.
>
> To cover both scenarios, introduce an X86CPU property that makes CPU
> init code to skip creation of the SynIC object (and thus disables any
> SynIC use in QEMU) but keeps the KVM part of the SynIC working.
> The property is clear by default but is set via compat logic for older
> machine types.
>
> As a result, when hv_synic and a modern machine type are specified, QEMU
> will refuse to run unless vp_index is on and the kernel is recent
> enough. OTOH with an older machine type QEMU will run fine with
> hv_synic=on against an older kernel and/or without vp_index enabled but
> will disallow the in-QEMU uses of SynIC (in e.g. VMBus).
>
> Signed-off-by: Roman Kagan <rka...@virtuozzo.com>
> ---
> include/hw/i386/pc.h | 8 ++++++++
> target/i386/cpu.h | 1 +
> target/i386/cpu.c | 2 ++
> target/i386/kvm.c | 30 ++++++++++++++++++++++--------
> 4 files changed, 33 insertions(+), 8 deletions(-)
>
> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> index 6894f37df1..dfe6746692 100644
> --- a/include/hw/i386/pc.h
> +++ b/include/hw/i386/pc.h
> @@ -294,6 +294,14 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
> int e820_get_num_entries(void);
> bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t *);
>
> +#define PC_COMPAT_3_0 \
> + HW_COMPAT_3_0 \
> + {\
> + .driver = TYPE_X86_CPU,\
> + .property = "x-hv-synic-kvm-only",\
> + .value = "on",\
> + }
Oops. This macro does nothing if the 3.1 machine-types aren't
updated to use it. This patch breaks compatibility on pc-*-3.1.
> [...]
--
Eduardo
