Re: SEV guest debugging support for Qemu

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Ashish Kalra (ashish.kalra@amd.com) wrote:
> Hello Alan, Paolo,
> 
> I am following up on Brijesh’s patches for SEV guest debugging support for Qemu using gdb and/or qemu monitor.
> I believe that last time, Qemu SEV debug patches were not applied and have attached the link to the email thread and Paolo’s feedback below for reference [1].
> I wanted to re-start a discussion on the same here with the Qemu community and seek the feedback on the approaches which we are considering :
> Looking at Qemu code, I see the following interface is defined, for virtual memory access for debug : cpu_memory_rw_debug(). 
> Both gdbstub (target_memory_rw_debug() ) and QMP/HMP (monitor/misc.c : memory_dump() ) use this standard and well-defined interface to access guest memory for debugging purposes. 
> 
> This internally invokes the address_space_rw() accessor functions which we had  "fixed" internally (as part of the earlier patch) to invoke memory region specific debug ops. 
> In our earlier approach we were adding debug ops/callbacks to memory regions and as per comments on our earlier patches, Paolo was not happy with this debug API for
> MemoryRegions and hence the SEV support for Qemu was merged without the debug support.
> 
> Now, we want to reuse this cpu_memory_rw_debug() interface or alternatively introduce a new generic debug interface/object in the Qemu. This 
> debug interface should be controlled through the global machine policy.

Let me leave the question of how the memory_rw_debug interface should
work to Paolo.

> For e.g., 
> # $QEMU -machine -debug=<a debug object>
> or
> # $QEMU -machine -debug=sev-guest-debug
> 
> The QMP and GDB access will be updated to use the generic debug  interface. The generic debug interface or the cpu_memory_rw_debug() interace will introduce hooks to call a 
> vendor specific debug object to delegate accessing the data. The vendor specific debug object may do a further checks before and after accessing the memory.

I'm not sure that needs a commandline switch for it; since you can
already get it from the guest policy in the sev object and I can't think
of any other cases that would need something similar.

> Now, looking specifically at cpu_memory_rw_debug() interface, this interface is invoked for all guest memory accesses for debugging purposes and it also does 
> guest VA to GPA translation via cpu_get_phys_page_attrs_debug(), so we can again add a vendor specific callback here to do guest VA to GPA translations specific
> to SEV as SEV guest debugging will also require accessing guest page table entries and decrypting them via the SEV DBG_DECRYPT APIs and additionally clearing
> the C-bit on page table entries (PxEs) before using them further for page table walks.
> 
> There is still an issue with the generic cpu_memory_rw_debug() interface, though it is used for all guest memory accesses for debugging and we can also handle
> guest page table walks via it (as mentioned above), there are still other gdb/monitor commands such as tlb_info_xx() and mem_info_xx() which also do guest page
> table walks, but they don’t go through any generic guest memory access/debug interface, so these commands will need to be handled additionally for SEV.

If some of those should be using the debug interface and aren't then
please fix them anyway.

> The vendor specific debug object (added as a hook to generic debug object or the generic cpu_memory_rw_debug() interface) will do further checks before and after accessing the memory.
> 
> e.g., in the case of SEV,
> 
> 1. Check the guest policy, if guest policy does not allow debug then return an error.
> 
> 2. If its an MMIO region then access the data.
> 
> 3. If its RAM region then call the PSP commands to decrypt the data.
> 
> 4. If caller asked to read the PTE entry then probably clear the C-bits after reading the PTE entry.

Does that work if the guest is currently running?

Dave

> 5. many more checks
> 
> Looking fwd. to your feedback/comments on the above approach or other any other suggestions.
> 
> Thanks,
> Ashish
> 
> [1] -> http://next.patchew.org/QEMU/20180308124901.83533-1-brijesh.singh@amd.com/20180308124901.83533-29-brijesh.singh@amd.com/
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK