Re: [Xen-devel] Question about VPID during MOV-TO-CR3

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.09.16 at 17:26, <tamas.lengyel@xxxxxxxxxxxx> wrote:
> On Fri, Sep 23, 2016 at 2:24 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>> On 22.09.16 at 19:18, <tamas.lengyel@xxxxxxxxxxxx> wrote:
>>> So I verified that when CPU-based load exiting is enabled, the TLB
>>> flush here is critical. Without it the guest kernel crashes at random
>>> points during boot. OTOH why does Xen trap every guest CR3 update
>>> unconditionally? While we have features such as the vm_event/monitor
>>> that may choose to subscribe to that event, Xen traps it even when
>>> that is not in use. Is that trapping necessary for something else?
>>
>> Where do you see this being unconditional? construct_vmcs()
>> clearly avoids setting these intercepts when using EPT. Are you
>> perhaps suffering from
>>
>>             /* Trap CR3 updates if CR3 memory events are enabled. */
>>             if ( v->domain->arch.monitor.write_ctrlreg_enabled &
>>                  monitor_ctrlreg_bitmask(VM_EVENT_X86_CR3) )
>>                 v->arch.hvm_vmx.exec_control |= CPU_BASED_CR3_LOAD_EXITING;
>>
>> in vmx_update_guest_cr()? That'll be rather something for you
>> or Razvan to explain. Outside of nested VMX I don't see any
>> other enabling of that intercept (didn't check AMD code on the
>> assumption that you're working on Intel hardware).
> 
> So there seems to be two separate paths that lead to the TLB flushing.
> One is indeed the above case you cited when we enable CR3 monitoring
> through the monitor interface. However, during domain boot I also see
> this path being called that is not related to the
> CPU_BASED_CR3_LOAD_EXITING:
> 
> (XEN) hap.c:739:d1v0 hap_update_paging_modes is calling hap_update_cr3
> (XEN) hap.c:701:d1v0 HAP update cr3 called
> (XEN) /src/xen/xen/include/asm/hvm/hvm.h:344:d1v0 HVM update guest cr3 called
> (XEN) vmx.c:1549:d1v0 Update guest CR3 value=0x7a7c4000
> 
> This path seems to de-activate once the domain is fully booted.

This late? According to the CR0 handling in
vmx_update_guest_cr() I would understand it to be enabled only
while the guest is still in real mode (and even then only on old
hardware, i.e. without the Unrestricted Guest functionality).

Jan


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