KVM: x86 emulator: Handle wraparound in (cs_base + offset) when fetching insns (0521e4c0) · Commits · 戴 / test

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:

Nelson Elhage <nelhage@ksplice.com> Signed-off-by:

Avi Kivity <avi@redhat.com>

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;

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