Re: [PATCH 1/10] Trivial: /dev/kvm interface is no longer experimental.

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Arnd Bergmann wrote:
> The equivalent of the current set of ioctls comes down to roughtly this
> set of syscalls:
>
> int kvm_create_vm(void);
> int kvm_get_msr_index_list(int fd, sttuct kvm_msr_list);
> size_t kvm_get_vcpu_mmap_size(int fd);
> int kvm_set_memory_region(int fd, unsigned slot, unsigned flags,
> 	__u64 guest_phys_addr, __u64 size);
> int kvm_create_vcpu(int fd);
> int kvm_get_dirty_log(int fd, unsigned slot, void *dirty_bitmap);
> int kvm_get_memory_alias(int fd, struct kvm_memory_region *region);
> int kvm_run(int fd);
> int kvm_get_regs(int fd, struct kvm_regs *regs);
> int kvm_set_regs(int fd, const struct kvm_regs *regs);
> int kvm_get_sregs(int fd, struct kvm_sregs *sregs);
> int kvm_set_sregs(int fd, const struct kvm_sregs *sregs);
> int kvm_translate(int fd, __u64 linear_address, __u64 *physical_address,
> 		__u8  *valid, __u8 *writeable, __u8 *usemode);
> int kvm_interrupt(int fd, __u32 irq);
> int kvm_debug_guest(...);
> int kvm_get_msrs(...);
> int kvm_set_msrs(...);
> int kvm_set_cpuid(...);
> int kvm_set_signal_mask(...);
> int kvm_get_fpu(...);
> int kvm_set_fpu(...);
>
> That's a lot of system calls! The only ioctl calls that can immediately
> go away are KVM_GET_API_VERSION and KVM_CHECK_EXTENSION, if the API
> is fixed.
>   

Many can be merged (set_fpu, set_regs, set_sregs).  We will need
CHECK_EXTENSION as long as we are unable to predict the future (well,
for syscalls that are intended to be invoked on initialization only we
can use ENOSYS).

> Before moving to the final syscall interface, I'd wait for the memory
> model to have moved to using a region of the host process address instead
> of a separate address range. As I understood Carsten, that's what was
> discussed in Ottawa anyway. It can probably remove a few of the
> existing ioctl calls.
>   

We will also wait until we get most ports working, so we get a chance to
test it in real life.

> Some more can be saved by changing the interface for the kvm_get/set_*
> calls. One way I can see this done at the syscall level is to replace
> kvm_get_regs() with
>
> regsfd = openat(vcpu, "regs", O_RDWR);
> (void)read(regsfs, &regs, sizeof (regs);
>
> Some of the others can also be done with files like this, e.g. writing
> to an "interrupt" file. Of course, you would keep the file descriptors
> open for the life time of the guest.
>   

Once we're back to using fds, we might as well use ioctls.  If anything,
an ioctl has an explicit mention of the structure it manipulates in its
definition.  I don't care that it came in last in the last 15 annual
kernel beauty contests.

For me, the difference between syscalls and ioctls is whether the vcpu
is bound to a task (and the vm bound to the mm) rather than whether the
names are spelled in lowercase or uppercase.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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