Re: [libvirt] cpu flags
On Wed, Sep 17, 2008 at 10:42:00AM +0100, Daniel P. Berrange wrote:
On Tue, Sep 16, 2008 at 04:45:09PM -0400, Ben Guthro wrote:
> Hi,
>
> We're finding that we are going to be needing the cpu flags (as reported
> in /proc/cpuinfo)
> ..specifically to find out if we are a vmx enabled machine.
>
> So - off I went looking into this for a patch to submit upstream.
> Unfortunately, I ran into some questions which need answering before I
> really proceed with this
>
> It seems to me that this info would best be parsed in src/nodeinfo.c
> This is where other cpuinfo things are parsed...and stored in the
> nodeinfo struct
> Perhaps we store this as a bitmask encoded int, as defined in
> /usr/include/asm/cpufeature.h and tack this onto the end of sad struct.
>
> My concern is that adding to the nodeinfo struct breaks the API - such
> that the structs will be different sizes between versions.
Would it be possible to hack around this? For example you could use
a unused bit of the old structure to set a flag for 'new-type-of-struct'.
The old libvirt would ignore it, the new one would look for it.
For example if it was an int enty and it was used as a flag entry wherein
only 4 bits would be used, you could use the 5th bit that would
tell it: "this is version 2 of the struct" and then the libvirt
would cast it to the new struct which would contain the extra data-bits.
This is how it is done in the RHEL kernels to work-around the kABI.