Re: [PATCH 1/13] KVM: userspace interface

[Avi Kivity]

Arnd Bergmann wrote:
> On Thursday 26 October 2006 19:22, Avi Kivity wrote:
>   
> > +
> > +/* for KVM_RUN */
> > +struct kvm_run {
> > +       /* in */
> > +       __u32 vcpu;
> > +       __u32 emulated;  /* skip current instruction */
> > +       __u32 mmio_completed; /* mmio request completed */
> > +
> > +       /* out */
> > +       __u32 exit_type;
> > +       __u32 exit_reason;
> > +       __u32 instruction_length;
> > +       union {
> > +               /* KVM_EXIT_UNKNOWN */
> > +               struct {
> > +                       __u32 hardware_exit_reason;
> > +               } hw;
> > +               /* KVM_EXIT_EXCEPTION */
> > +               struct {
> > +                       __u32 exception;
> > +                       __u32 error_code;
> > +               } ex;
> > +               /* KVM_EXIT_IO */
> > +               struct {
> > +#define KVM_EXIT_IO_IN  0
> > +#define KVM_EXIT_IO_OUT 1
> > +                       __u8 direction;
> > +                       __u8 size; /* bytes */
> > +                       __u8 string;
> > +                       __u8 string_down;
> > +                       __u8 rep;
> > +                       __u8 pad;
> > +                       __u16 port;
> > +                       __u64 count;
> > +                       union {
> > +                               __u64 address;
> > +                               __u32 value;
> > +                       };
> > +               } io;
> > +               struct {
> > +               } debug;
> > +               /* KVM_EXIT_MMIO */
> > +               struct {
> > +                       __u64 phys_addr;
> > +                       __u8  data[8];
> > +                       __u32 len;
> > +                       __u8  is_write;
> > +               } mmio;
> > +       };
> > +};
> >     
>
> This data structure looks like it can become maintenance problem. It's
> already sufficiently complex that I would assume that you need to extend
> it in the future even further, which is hard to do in a compatible
> way. How confident are you that, for a given exit reason, you don't need
> to add or extend the existing fields further, thereby breaking the ABI?
> Do you have a plan how to deal with such breakage?
>
>   

I guess we can reserve some extra space, and if we need even more space 
a new exit reason can specify a pointer to receive the exit data.

Of course, userspace should not get an unexpected exit reason.  I'm not 
worried about the short term, since it's likely there will only be a 
single user (modified qemu), long term we'll have userspace enable new 
features explicitly.

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

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