Re: [PATCH 3/3] KVM: Expose MSRs to userspace

Arnd Bergmann
2006-11-16T19:08:48Z

On Thursday 16 November 2006 19:04, Avi Kivity wrote:
> +struct kvm_msr_entry {
> +       __u32 index;
> +       __u32 reserved;
> +       __u64 data;
> +};
> +
> +/* for KVM_GET_MSRS and KVM_SET_MSRS */
> +struct kvm_msrs {
> +       __u32 vcpu;
> +       __u32 nmsrs; /* number of msrs in entries */
> +
> +       union {
> +               struct kvm_msr_entry __user *entries;
> +               __u64 padding;
> +       };
> +};

ioctl interfaces with pointers in them are generally a bad idea,
though you handle most of the points against them fine here
(endianess doesn't matter, padding is correct).

Still, it might be better not to set a bad example. Is accessing
the MSRs actually performance critical? If not, you could
define the ioctl to take only a single entry argument.

A possible alternative could also be to have a variable length
argument like below, but that creates other problems:

+struct kvm_msrs {
+ __u32 vcpu;
+ __u32 nmsrs; /* number of msrs in entries */
+ struct kvm_msr_entry entries[0]; /* followed by actual msrs */
+};

This would mean that you can't tell the transfer size from the
ioctl number, but you can't do that in your code either, because
you do two separate transfers.