Re: [PATCH v4 1/2] powerpc/pseries/iommu: Share the per-cpu TCE page with the hypervisor.

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 05/12/2019 07:42, Ram Pai wrote:
> On Wed, Dec 04, 2019 at 02:36:18PM +1100, David Gibson wrote:
>> On Wed, Dec 04, 2019 at 12:08:09PM +1100, Alexey Kardashevskiy wrote:
>>>
>>>
>>> On 04/12/2019 11:49, Ram Pai wrote:
>>>> On Wed, Dec 04, 2019 at 11:04:04AM +1100, Alexey Kardashevskiy wrote:
>>>>>
>>>>>
>>>>> On 04/12/2019 03:52, Ram Pai wrote:
>>>>>> On Tue, Dec 03, 2019 at 03:24:37PM +1100, Alexey Kardashevskiy wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 03/12/2019 15:05, Ram Pai wrote:
>>>>>>>> On Tue, Dec 03, 2019 at 01:15:04PM +1100, Alexey Kardashevskiy wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 03/12/2019 13:08, Ram Pai wrote:
>>>>>>>>>> On Tue, Dec 03, 2019 at 11:56:43AM +1100, Alexey Kardashevskiy wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On 02/12/2019 17:45, Ram Pai wrote:
>>>>>>>>>>>> H_PUT_TCE_INDIRECT hcall uses a page filled with TCE entries, as one of
>>>>>>>>>>>> its parameters. One page is dedicated per cpu, for the lifetime of the
>>>>>>>>>>>> kernel for this purpose. On secure VMs, contents of this page, when
>>>>>>>>>>>> accessed by the hypervisor, retrieves encrypted TCE entries.  Hypervisor
>>>>>>>>>>>> needs to know the unencrypted entries, to update the TCE table
>>>>>>>>>>>> accordingly.  There is nothing secret or sensitive about these entries.
>>>>>>>>>>>> Hence share the page with the hypervisor.
>>>>>>>>>>>
>>>>>>>>>>> This unsecures a page in the guest in a random place which creates an
>>>>>>>>>>> additional attack surface which is hard to exploit indeed but
>>>>>>>>>>> nevertheless it is there.
>>>>>>>>>>> A safer option would be not to use the
>>>>>>>>>>> hcall-multi-tce hyperrtas option (which translates FW_FEATURE_MULTITCE
>>>>>>>>>>> in the guest).
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Hmm... How do we not use it?  AFAICT hcall-multi-tce option gets invoked
>>>>>>>>>> automatically when IOMMU option is enabled.
>>>>>>>>>
>>>>>>>>> It is advertised by QEMU but the guest does not have to use it.
>>>>>>>>
>>>>>>>> Are you suggesting that even normal-guest, not use hcall-multi-tce?
>>>>>>>> or just secure-guest?  
>>>>>>>
>>>>>>>
>>>>>>> Just secure.
>>>>>>
>>>>>> hmm..  how are the TCE entries communicated to the hypervisor, if
>>>>>> hcall-multi-tce is disabled?
>>>>>
>>>>> Via H_PUT_TCE which updates 1 entry at once (sets or clears).
>>>>> hcall-multi-tce  enables H_PUT_TCE_INDIRECT (512 entries at once) and
>>>>> H_STUFF_TCE (clearing, up to 4bln at once? many), these are simply an
>>>>> optimization.
>>>>
>>>> Do you still think, secure-VM should use H_PUT_TCE and not
>>>> H_PUT_TCE_INDIRECT?  And normal VM should use H_PUT_TCE_INDIRECT?
>>>> Is there any advantage of special casing it for secure-VMs.
>>>
>>>
>>> Reducing the amount of insecure memory at random location.
>>
>> The other approach we could use for that - which would still allow
>> H_PUT_TCE_INDIRECT, would be to allocate the TCE buffer page from the
>> same pool that we use for the bounce buffers.  I assume there must
>> already be some sort of allocator for that?
> 
> The allocator for swiotlb is buried deep in the swiotlb code. It is 
> not exposed to the outside-swiotlb world. Will have to do major surgery
> to expose it.
> 
> I was thinking, maybe we share the page, finish the INDIRECT_TCE call,
> and unshare the page.  This will address Alexey's concern of having
> shared pages at random location, and will also give me my performance
> optimization.  Alexey: ok?


I really do not see the point. I really think we should to 1:1 mapping
of swtiotlb buffers using the default 32bit window using H_PUT_TCE and
this should be more than enough, I do not think the amount of code will
be dramatically different compared to unsecuring and securing a page or
using one of swtiotlb pages for this purpose. Thanks,


-- 
Alexey