Re: [RFC PATCH v3 12/36] i386/tdx: Add property sept-ve-disable for tdx-guest object

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/24/2022 3:57 PM, Gerd Hoffmann wrote:
> On Thu, Mar 24, 2022 at 02:52:10PM +0800, Xiaoyao Li wrote:
>> On 3/22/2022 5:02 PM, Gerd Hoffmann wrote:
>>> On Thu, Mar 17, 2022 at 09:58:49PM +0800, Xiaoyao Li wrote:
>>>> Add sept-ve-disable property for tdx-guest object. It's used to
>>>> configure bit 28 of TD attributes.
>>>
>>> What is this?
>>
>> It seems this bit doesn't show up in the public spec yet.
>>
>> Bit 28 (SEPT_VE_DISABLE): Disable EPT violation conversion to #VE ON guest
>> TD ACCESS of PENDING pages.
>>
>> The TDX architecture requires a private page to be accepted before using. If
>> guest accesses a not-accepted (pending) page it will get #VE.
>>
>> For some OS, e.g., Linux TD guest, it doesn't want the #VE on pending page
>> so it will set this bit.
> 
> Hmm.  That looks rather pointless to me.  The TDX patches for OVMF add a
> #VE handler, so I suspect every guest wants #VE exceptions if even the
> firmware cares to install a handler ...

#VE can be triggered in various situations. e.g., CPUID on some leaves, 
and RD/WRMSR on some MSRs. #VE on pending page is just one of the 
sources, Linux just wants to disable this kind of #VE since it wants to 
prevent unexpected #VE during SYSCALL gap.

> Also: What will happen instead? EPT fault delivered to the host?

Yes.

> take care,
>    Gerd
>