Re: [PATCH 0/1] ARM: Implement support for write-ctrlreg vm-events

[Corneliu ZUZU <czuzu@bitdefender.com>]

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On 3/7/2016 2:38 PM, Andrew Cooper wrote:
> On 07/03/16 09:12, Tamas K Lengyel wrote:
>>
>>
>> On Mon, Mar 7, 2016 at 9:22 AM, Corneliu ZUZU <czuzu@bitdefender.com 
>> <mailto:czuzu@bitdefender.com>> wrote:
>>
>>     On 3/3/2016 4:10 PM, Corneliu ZUZU wrote:
>>
>>         Then,
>>         QUESTIONS (FOR VM-EVENTS & ARM MAINTAINERS ESPECIALLY):
>>
>>         Q1) [...]
>>
>>         Q2) [...]
>>
>>         Q3) [...]
>>
>>         Q4) [...]
>>
>>
>>     Hey all,
>>
>>     I have a question relating to this part of code @
>>     vmx_update_guest_cr:
>>
>>             if ( paging_mode_hap(v->domain) )
>>             {
>>                 /* Manage GUEST_CR3 when CR0.PE <http://CR0.PE>=0. */
>>                 uint32_t cr3_ctls = (CPU_BASED_CR3_LOAD_EXITING |
>>      CPU_BASED_CR3_STORE_EXITING);
>>                 v->arch.hvm_vmx.exec_control &= ~cr3_ctls;
>>                 if ( !hvm_paging_enabled(v) &&
>>     !vmx_unrestricted_guest(v) )
>>                     v->arch.hvm_vmx.exec_control |= cr3_ctls;
>>
>>                 /* 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;
>>
>>                 vmx_update_cpu_exec_control(v);
>>             }
>>
>>     While trying to move the check for VM_EVENT_X86_CR3 to the
>>     scheduling tail, a few questions came to my mind.
>>
>>     1). Tamas, Razvan, maybe you guys could clarify this. I noticed
>>     this part of code is only executed if paging_mode_hap(v->domain).
>>     Is EPT mandatory to monitor CR3 writes or is it just that when
>>     shadow paging is enabled, CR3 r/w are unconditionally trapped?
>>
>>
>> EPT is not really required for CR3 monitoring, it just has been the 
>> case that vm_events have been only implemented for hap-enabled 
>> domains. AFAIK for non-hap case CR3 needs to be trapped 
>> unconditionally, yes.
>
> Specifically, the shadow pagetable code needs to swap shadows when the 
> guest switches cr3.
>
>>     If the former is true, shouldn't we do a check like this in
>>     vm_event_monitor_get_capabilities instead?
>>
>>
>> Yes, it should now, this code was just written before 
>> vm_event_monitor_get_capabilities was introduced and we haven't 
>> gotten around converting this check to it.
>>
>>
>>     2). I was also wondering why CR3 load/stores are trapped if
>>     paging is disabled for a domain.
>>
>>
>> Good question, I was wondering about that myself at some point but I 
>> haven't found an answer to it. Maybe some git archaeology can help 
>> determining when that was added and why ;)
>
> Gen1 VT-x didn't support running a guest in non-paged mode.  Gen2 
> introduced "unrestricted-guest" which works as intended, but Gen1 has 
> to fake non-pagad mode using identity paging.  As a result, CR3 cannot 
> be used as scratch space like it can in non-paged mode, and the guest 
> must be prevented from moving CR3 away from the gfn set up by the 
> domain builder in HVM_PARAM_IDENT_PT.
>
> ~Andrew

Nice, thanks a bunch.

Corneliu.

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