Re: SEV guest debugging support for Qemu

[Ashish Kalra <ashish.kalra@amd.com>]

On Thu, Sep 24, 2020 at 02:37:33PM -0500, Brijesh Singh wrote:
> 
> On 9/24/20 2:06 PM, Ashish Kalra wrote:
> > Hello Dave,
> >
> > Thanks for your response, please see my replies inline :
> >
> > On Thu, Sep 24, 2020 at 02:53:42PM +0100, Dr. David Alan Gilbert wrote:
> >> * 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.
> > Yes, i agree with that, so i am now considering abstracting this vendor
> > specific debug interface via CPUClass object instead of doing it via
> > MemoryRegions. 
> >
> >>> 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?
> >>
> > I assume you are asking that is this done when guest is being debugged,
> > the above steps are only done when the guest is paused and being debugged.
> 
> 
> I don't why we need to pause the guest. Ideally we should be able to
> connect to Qemu monitor and run the "x" command to dump memory. IIRC, if
> paging is enabled then monitor will walk the guest page table to reach
> to gpa. Something like this in the Qemu monitor console should work:
> 
> x /10i $eip
> 
> 

Yes that works, what i basically meant that monitor will invoke a set of debugging
interfaces to get gpa and then dump guest memory even while guest is
running.

Thanks,
Ashish

> >
> >>> 5. many more checks
> >>>
> >>> Looking fwd. to your feedback/comments on the above approach or other any other suggestions.
> >>>
> >>> Thanks,
> >>> Ashish
> >>>
> >>> [1] -> https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fnext.patchew.org%2FQEMU%2F20180308124901.83533-1-brijesh.singh%40amd.com%2F20180308124901.83533-29-brijesh.singh%40amd.com%2F&amp;data=02%7C01%7Cashish.kalra%40amd.com%7Cd21e40d3527d4dba609c08d86091490e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637365524404435805&amp;sdata=P%2F6DqPQmUObJipkbbeXcrUdCqulePiqxSU6OB8xUEWo%3D&amp;reserved=0
> >>>
> >> -- 
> >> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >>