Re: [PATCH 4/5] kvmtool: Save datamatch as little endian in {add,del}_event

[Andreas Herrmann <andreas.herrmann@caviumnetworks.com>]

On Tue, Jun 16, 2015 at 06:17:14PM +0100, Will Deacon wrote:
> On Mon, Jun 15, 2015 at 12:49:45PM +0100, Andreas Herrmann wrote:
> > W/o dedicated endianess it's impossible to find reliably a match
> > e.g. in kernel/virt/kvm/eventfd.c ioeventfd_in_range.
> 
> Hmm, but shouldn't this be the endianness of the guest, rather than just
> forcing things to little-endian?

With my patch and following adaption to
ioeventfd_in_range (in virt/kvm/eventfd.c):

        switch (len) {
	case 1:
                _val = *(u8 *)val;
                break;
        case 2:
                _val = le16_to_cpu(*(u16 *)val);
                break;
	case 4:
                _val = le32_to_cpu(*(u32 *)val);
                break;
        case 8:
                _val = le64_to_cpu(*(u64 *)val);
		        break;
        default:
                return false;
		}

        return _val == le64_to_cpu(p->datamatch) ? true : false;

datamatch is properly evaluated on either endianess.

The current code in ioeventfd_in_range looks fragile to me (for big
endian systems) and didn't work with kvmtool:

        switch (len) {
        case 1:
                _val = *(u8 *)val;
                break;
        case 2:
                _val = *(u16 *)val;
                break;
        case 4:
                _val = *(u32 *)val;
                break;
        case 8:
                _val = *(u64 *)val;
                break;
        default:
                return false;
        }

        return _val == p->datamatch ? true : false;

But now I see, w/o a correponding kernel change the patch shouldn't
be merged.


Andreas