On Thu, Jun 23, 2016 at 06:03:01PM +0200, Paolo Bonzini wrote:

On 18/06/2016 11:01, Quentin Casasnovas wrote:

quoted

Cross-checking the KVM/VMX VMREAD emulation code with the Intel Software
Developper Manual Volume 3C - "VMREAD - Read Field from Virtual-Machine
Control Structure", I found that we're enforcing that the destination
operand is NOT located in a read-only data segment or any code segment when
the L1 is in long mode - BUT that check should only happen when it is in
protected mode.

Shuffling the code a bit to make our emulation follow the specification
allows me to boot a Xen dom0 in a nested KVM and start HVM L2 guests
without problems.

That's great, and I'm applying the patch, but it's also pretty weird. :)
 Do you have a pointer to Xen source code that does a VMREAD into a
read-only data segment or a code segment?

It is indeed pretty weird.  Looking at the Xen stack trace, it looks like
the vmread is writing to an on-stack buffer, and surely it must be writable
so I wonder if Xen might not be using an executable stack for some reason?
That would be a bit scary so I'm surely missing something.

Is there an easy way to know from my KVM host the different segment
permission setup by the guest?

Quentin