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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