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Re: [Xen-devel] Re: [RFC PATCH 18/35] Support gdt/idt/ldt handling on Xen.

Andi Kleen wrote:
On Wednesday 22 March 2006 07:30, Chris Wright wrote:

-#define load_TR_desc() __asm__ __volatile__("ltr %w0"::"q" (GDT_ENTRY_TSS*8))
-#define load_LDT_desc() __asm__ __volatile__("lldt %w0"::"q" (GDT_ENTRY_LDT*8))
-#define load_gdt(dtr) __asm__ __volatile("lgdt %0"::"m" (*dtr))
-#define load_idt(dtr) __asm__ __volatile("lidt %0"::"m" (*dtr))
-#define load_tr(tr) __asm__ __volatile("ltr %0"::"mr" (tr))
-#define load_ldt(ldt) __asm__ __volatile("lldt %0"::"mr" (ldt))
-#define store_gdt(dtr) __asm__ ("sgdt %0":"=m" (*dtr))
-#define store_idt(dtr) __asm__ ("sidt %0":"=m" (*dtr))
-#define store_tr(tr) __asm__ ("str %0":"=mr" (tr))
-#define store_ldt(ldt) __asm__ ("sldt %0":"=mr" (ldt))

These are all very infrequent except perhaps LLDT. I suspect trapping would work too. But ok.

Yes, trapping works fine. Even LLDT is infrequent. But, you do impose a very large amount of complexity on the hypervisor by trapping on any instruction. Suppose you wanted to write a minimal hypervisor, which consisted of pretty much a wrapper based on the Xen or VMI interface that just stole a couple pages of physical memory, and hooked trap handlers by hooking the call out for lidt.

If you instead remove the hypervisor wrapper for lidt, and require the hypervisor to trap and emulate it, you have just imposed an insidious amount of overhead on it. It doesn't seem too much at first - trap and emulate, right?

No. First, you have to create a special #GP handler for the general protection fault. But the fault doesn't tell you anything about why it happened - just that it was a general protection fault, and maybe a segment related to it. To figure out what happened, you have to decode the instruction stream. To decode the instruction stream, you have to take the EIP pointer and read from it, right? Wrong. You have to extract segment information from the code segment, apply segmentation rules to the access, rule out invalid processor modes, deal with wrap around conditions, etc. But lets say you do all that. Now, you have to read the guest memory to decode. Which requires reading guest page tables. The memory in question has to be mapped present and executable. You have to deal conditionally with PAE / non-PAE paging modes. And race conditions where self-modifying code can trick you into decoding something that really didn't happen. Then, finally, you can interpret the instruction, go through the whole process of reading guest memory again (fortunately, this time, without segmentation), read the guest IDT, and hook in your trap handlers where appropriate.

Yeah, it really is that bad, and that is really only a scratch on the surface. Trap and emulate, while possible, is basically about as evil a requirement as you can impose on a hypervisor. Everyone who is serious about the market needs to support it in one form or another eventually, but it raises the bar to such a point as to stop proliferation of minimal hypervisors, which could make extremely useful research tools for the community under an open source license.


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