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.ka...@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. 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&data=02%7C01%7Cashish.kalra%40amd.com%7Cd21e40d3527d4dba609c08d86091490e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637365524404435805&sdata=P%2F6DqPQmUObJipkbbeXcrUdCqulePiqxSU6OB8xUEWo%3D&reserved=0 > > > -- > Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK >
