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Re: [Xen-devel] [PATCH for-4.7] x86/pv: Correctly fold vIOPL back into vcpu_guest_context

On 04/11/2016 05:03 AM, Andrew Cooper wrote:
c/s f71ecb6 "x86: introduce a new VMASSIST for architectural behaviour of
iopl" shifted the vcpu iopl field by 12, but didn't update the logic which
reconstructs the guests eflags for migration.

Existing guest kernels set a vIOPL of 1, to prevent them from faulting when
accessing IO ports.  This bug manifests as a crash after migrate, as the vIOPL
reverts back to the default of 0, and the guest suffers an unexpected #GP

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Tested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

(save/restore was part of brokenness as well, obviously)

CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
  xen/arch/x86/domctl.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
index cba1e37..6e709cb 100644
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -1512,7 +1512,7 @@ void arch_get_info_guest(struct vcpu *v, 
vcpu_guest_context_u c)
/* IOPL privileges are virtualised: merge back into returned eflags. */
          BUG_ON((c(user_regs.eflags) & X86_EFLAGS_IOPL) != 0);
-        c(user_regs.eflags |= v->arch.pv_vcpu.iopl << 12);
+        c(user_regs.eflags |= v->arch.pv_vcpu.iopl);
if ( !compat )

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