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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: Nelson Elhage <nelhage@ksplice.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
This commit is contained in:
Nelson Elhage 2011-04-13 11:44:13 -04:00 committed by Avi Kivity
parent 49704f2658
commit 0521e4c0bc
1 changed files with 4 additions and 1 deletions
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5
arch/x86/kvm/emulate.c
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@ -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;