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

[Ram Pai <linuxram@us.ibm.com>]

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?

RP