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