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.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. > > > 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&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 > >>
