Re: [RFC PATCH v2 03/22] i386/xen: Add xen-version machine property and init KVM Xen support

[Paolo Bonzini]

On 12/9/22 10:55, David Woodhouse wrote:
> -    m->default_machine_opts = "accel=xen,suppress-vmdesc=on";
> +    if (xen_enabled())
> +            m->default_machine_opts = "accel=xen,suppress-vmdesc=on";
> +    else
> +            m->default_machine_opts = "accel=kvm,xen-version=0x30001";

Please do not modify pc_xen_hvm_init().

"-M xenfv" should be the same as "-M pc-i440fx-...,suppress-vmdesc=on 
-accel xen -device xen-platform".  It must work *without* "-accel xen", 
while here you're basically requiring it.  For now, please make 
KVM-emulated Xen use "-M pc -device xen-platform".  We can figure out 
"-M xenfv" later.

You can instead have:

- a check in xen_init() that checks that xen_mode is XEN_ATTACH.  If 
not, fail.

- an extra enum value for xen_mode, XEN_DISABLED, which is the default 
instead of XEN_EMULATE;

- an accelerator property "-accel kvm,xen-version=...", added in 
kvm_accel_class_init() instead of the machine property.  The property, 
when set to a nonzero value, flips xen_mode from XEN_DISABLED to 
XEN_EMULATE.

The Xen overlay device can be created using the mc->kvm_type function 
(which you can set in DEFINE_PC_MACHINE); at that point, xen_mode has 
switched from XEN_DISABLED to XEN_EMULATE.  Those xen_enabled() checks 
that apply to KVM then become xen_mode != XEN_DISABLED, as long as they 
run during mc->kvm_type or afterwards.

The platform device can be created either in mc->kvm_type or manually 
(not sure if it makes sense to have a "XenVMMXenVMM" CPUID + emulated 
hypercalls but no platform device---would it still use pvclock for 
example?).

Paolo