Re: [PATCH for-1.4 04/12] kvm: Create kvm_arch_vcpu_id() function

[Eric Blake <eblake@redhat.com>]

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On 01/18/2013 07:20 AM, Eduardo Habkost wrote:
>> Could you suggest a text for me to add please?
> 
> "The argument passed to KVM_CREATE_VCPU now has 'unsigned long' type
> instead of 'int', as expected by the Linux ioctl() syscall. Maybe an int
> works on most or all architectures supporting KVM, but it is safer to
> use an appropriate 'unsigned long' parameter."

Interestingly enough, while the Linux syscall uses 'unsigned long', the
POSIX definition of ioctl() uses 'int'; so the Linux kernel is already
constrained to never use an ioctl value that doesn't fit within 'int',
and glibc is already responsible for ensuring that argument promotion of
an int doesn't change the behavior of ioctl() in libc when converting it
over to the unsigned long syscall semantics expected by the kernel.

> 
> To find out if 'int' breaks on any architecture, I would need to check
> the ABI specification for each architecture. I didn't do that, but I am
> sure we should pass an unsigned long instead, if that's the type
> expected by the kernel.
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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