On 06/30/2011 05:16 PM, Avi Kivity wrote:

On 06/30/2011 06:04 PM, Alexander Graf wrote:

quoted

quoted

+4.64 KVM_PPC_SET_PLATFORM
+
+Capability: none
+Architectures: powerpc
+Type: vm ioctl
+Parameters: struct kvm_ppc_set_platform (in)
+Returns: 0, or -1 on error
+
+This is used by userspace to tell KVM what sort of platform it should
+emulate.  The return value of the ioctl tells userspace whether the
+emulation it is requesting is supported by KVM.
+
+struct kvm_ppc_set_platform {
+    __u16 platform;        /* defines the OS/hypervisor ABI */
+    __u16 guest_arch;    /* e.g. decimal 206 for v2.06 */
+    __u32 flags;

Please add some padding so we can extend it later if necessary.

Regarding that.  There's another option - the ioctl code embeds the 
structure size.  So if we extend the ioctl parsing to pad up (or 
truncate down) from the user's size to our size, and similarly in the 
other direction, we can get away from this ugliness.

Some years ago I posted a generic helper that did this (and also 
kmalloc'ed and kfree'd the data itself), but it wasn't received 
favourably.  Maybe I should try again (and we can possibly use it in 
kvm even if it is rejected for general use, though that's against our 
principles of pushing all generic infrastructure to the wider kernel).

That does sound interesting, but requires a lot more thought to be put 
into the actual code, as we basically need to read out the feature 
bitmap, then provide a minimum size for the chosen features and then 
decide if they fit in.

But yes, that'd still be a lot better than shoving around useless data 
all the time :)


Alex