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Commit 0521e4c0 authored by Nelson Elhage's avatar Nelson Elhage Committed by Avi Kivity
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KVM: x86 emulator: Handle wraparound in (cs_base + offset) when fetching insns 



Currently, setting a large (i.e. negative) base address for %cs does not work on
a 64-bit host. The "JOS" teaching operating system, used by MIT and other
universities, relies on such segments while bootstrapping its way to full
virtual memory management.

Signed-off-by: default avatarNelson Elhage <nelhage@ksplice.com>
Signed-off-by: default avatarAvi Kivity <avi@redhat.com>
parent 49704f26

arch/x86/kvm/emulate.c

+4 −1
Original line number Diff line number Diff line
@@ -637,9 +637,12 @@ static int do_fetch_insn_byte(struct x86_emulate_ctxt *ctxt, 
	int size, cur_size;



	if (eip == fc->end) {

		unsigned long linear = eip + ctxt->cs_base;

		if (ctxt->mode != X86EMUL_MODE_PROT64)

			linear &= (u32)-1;

		cur_size = fc->end - fc->start;

		size = min(15UL - cur_size, PAGE_SIZE - offset_in_page(eip));

		rc = ops->fetch(ctxt->cs_base + eip, fc->data + cur_size,

		rc = ops->fetch(linear, fc->data + cur_size,

				size, ctxt->vcpu, &ctxt->exception);

		if (rc != X86EMUL_CONTINUE)

			return rc;