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 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.
In fact, we could make use of as much optimization as possible.


> 
> >>>> Is not this for pci+swiotlb? 
..snip..
> >>> This patch is purely to help the hypervisor setup the TCE table, in the
> >>> presence of a IOMMU.
> >>
> >> Then the hypervisor should be able to access the guest pages mapped for
> >> DMA and these pages should be made unsecure for this to work. Where/when
> >> does this happen?
> > 
> > This happens in the SWIOTLB code.  The code to do that is already
> > upstream.  
> >
> > The sharing of the pages containing the SWIOTLB bounce buffers is done
> > in init_svm() which calls swiotlb_update_mem_attributes() which calls
> > set_memory_decrypted().  In the case of pseries, set_memory_decrypted() calls 
> > uv_share_page().
> 
> 
> This does not seem enough as when you enforce iommu_platform=on, QEMU
> starts accessing virtio buffers via IOMMU so bounce buffers have to be
> mapped explicitly, via H_PUT_TCE&co, where does this happen?
> 

I think, it happens at boot time. Every page of the guest memory is TCE
mapped, if iommu is enabled. SWIOTLB pages get implicitly TCE-mapped
as part of that operation.

RP