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Re: [Xen-devel] [PATCH RFC v12 16/21] pvh: Use PV handlers for emulated forced invalid ops, cpuid, and IO

On 20/09/13 18:03, George Dunlap wrote:
On 19/09/13 11:09, Jan Beulich wrote:
On 19.09.13 at 03:02, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:
On Wed, 18 Sep 2013 16:31:17 +0100 "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
On 13.09.13 at 18:25, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
@@ -1624,6 +1631,13 @@ static int guest_io_okay(
      int user_mode = !(v->arch.flags & TF_kernel_mode);
  #define TOGGLE_MODE() if ( user_mode ) toggle_guest_mode(v)
  +    /*
+     * For PVH we check this in vmexit for
+     * and so don't need to check again here.
+     */
+    if ( is_pvh_vcpu(v) )
+        return 1;
      if ( !vm86_mode(regs) &&
           (v->arch.pv_vcpu.iopl >= (guest_kernel_mode(v, regs) ?
1 : 3)) ) return 1;
Hmm, am I missing something here? The check in the VMEXIT
handler is just a privilege level one - where's the bitmap being
consulted? _If_ the bitmap is being maintained properly for the
guest (which I don't recall having seen), anything leading here
would be for ports the guest was not permitted access to. Yet
we would happily emulate the access for it then.
Not sure I understand which bitmap needs to be consulted. The bitmap
hvm_io_bitmap is used to set the intercepts which PVH also uses, with
HVM defaults.
Actually I think I got confused by the IOPL checking in your
VM exit handling code: _If_ you need to check the IOPL, then
you also need to check the bitmap hanging off of the TSS. But
aiui all the privilege level checks get done inside the guest, and
only if those pass would the I/O bitmap hanging off of the VMCS
be consulted. Hence neither the IOPL check nor the TSS-based
bitmap check ought to be necessary here.

I'm not quite following this. Are you saying that the iopl checks are done by the hardware, and if failed it will already have delivered a GPF; and if we've managed to get an IO_INSTRUCTION exit then they have passed, so we don't need the iopl check in vmx.c?

It does look as if the HVM path doesn't do any checking at all.


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