Re: [libvirt] [PATCH v3 3/4] qemu_domain: NVLink2 device tree functions for PPC64

On 08/03/2019 04:51, Piotr Jaroszynski wrote: > On 3/7/19 7:39 AM, Daniel Henrique Barboza wrote: >> >> On 3/7/19 12:15 PM, Erik Skultety wrote: >>> On Tue, Mar 05, 2019 at 09:46:08AM -0300, Daniel Henrique Barboza wrote: >>>> The NVLink2 support in QEMU implements the detection of NVLink2 >>>> capable devices by verfying the attributes of the VFIO mem region >>>> QEMU allocates for the NVIDIA GPUs. To properly allocate an >>>> adequate amount of memLock, Libvirt needs this information before >>>> a QEMU instance is even created. >>>> >>>> An alternative is presented in this patch. Given a PCI device, >>>> we'll traverse the device tree at /proc/device-tree to check if >>>> the device has a NPU bridge, retrieve the node of the NVLink2 bus, >>>> find the memory-node that is related to the bus and see if it's a >>>> NVLink2 bus by inspecting its 'reg' value. This logic is contained >>>> inside the 'device_is_nvlink2_capable' function, which uses other >>>> new helper functions to navigate and fetch values from the device >>>> tree nodes. >>>> >>>> Signed-off-by: Daniel Henrique Barboza <danielhb413(a)gmail.com&gt; >>>> --- >>> This fails with a bunch of compilation errors, please make sure you run make >>> check and make syntax-check on each patch of the series. >> Ooops, forgot to follow up make syntax-check with 'make' before >> submitting ... my bad. >> >>>> src/qemu/qemu_domain.c | 194 +++++++++++++++++++++++++++++++++++++++++ >>>> 1 file changed, 194 insertions(+) >>>> >>>> diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c >>>> index 77548c224c..97de5793e2 100644 >>>> --- a/src/qemu/qemu_domain.c >>>> +++ b/src/qemu/qemu_domain.c >>>> @@ -10343,6 +10343,200 @@ qemuDomainUpdateCurrentMemorySize(virDomainObjPtr vm) >>>> } >>>> >>>> >>>> +/** >>>> + * Reads a phandle file and returns the phandle value. >>>> + */ >>>> +static int >>>> +read_dt_phandle(const char* file) >>> ..we prefer camelCase naming of functions...all functions should have a prefix, >>> e.g. vir,<driver>, etc. >>> >>>> +{ >>>> + unsigned int buf[1]; >>>> + size_t read; >>>> + FILE *f; >>>> + >>>> + f = fopen(file, "r"); >>>> + if (!f) >>>> + return -1; >>>> + >>>> + read = fread(buf, sizeof(unsigned int), 1, f); >>> We already have safe helpers that take care of such operations. >>> >>>> + >>>> + if (!read) { >>>> + VIR_CLOSE(f); >>> You could use VIR_AUTOCLOSE for this >>> >>>> + return 0; >>>> + } >>>> + >>>> + VIR_CLOSE(f); >>>> + return be32toh(buf[0]); >>> Is this part of gnulib? We need to make sure it's portable otherwise we'd need >>> to make the conversion ourselves, just like for le64toh >>> >>>> +} >>>> + >>>> + >>>> +/** >>>> + * Reads a memory reg file and returns the first 4 int values. >>>> + * >>>> + * The caller is responsible for freeing the returned array. >>>> + */ >>>> +static unsigned int * >>>> +read_dt_memory_reg(const char *file) >>>> +{ >>>> + unsigned int *buf; >>>> + size_t read, i; >>>> + FILE *f; >>>> + >>>> + f = fopen(file, "r"); >>>> + if (!f) >>>> + return NULL; >>>> + >>>> + if (VIR_ALLOC_N(buf, 4) < 0) >>>> + return NULL; >>>> + >>>> + read = fread(buf, sizeof(unsigned int), 4, f); >>>> + >>>> + if (!read && read < 4) >>>> + /* shouldn't happen */ >>>> + VIR_FREE(buf); >>>> + else for (i = 0; i < 4; i++) >>>> + buf[i] = be32toh(buf[i]); >>>> + >>>> + VIR_CLOSE(f); >>>> + return buf; >>> Same comments as above... >>> >>>> +} >>>> + >>>> + >>>> +/** >>>> + * This wrapper function receives arguments to be used in a >>>> + * 'find' call to retrieve the file names that matches >>>> + * the criteria inside the /proc/device-tree dir. >>>> + * >>>> + * A 'find' call with '-iname phandle' inside /proc/device-tree >>>> + * provides more than a thousand matches. Adding '-path' to >>>> + * narrow it down further is necessary to keep the file >>>> + * listing sane. >>>> + * >>>> + * The caller is responsible to free the buffer returned by >>>> + * this function. >>>> + */ >>>> +static char * >>>> +retrieve_dt_files_pattern(const char *path_pattern, const char *file_pattern) >>>> +{ >>>> + virCommandPtr cmd = NULL; >>>> + char *output = NULL; >>>> + >>>> + cmd = virCommandNew("find"); >>>> + virCommandAddArgList(cmd, "/proc/device-tree/", "-path", path_pattern, >>>> + "-iname", file_pattern, NULL); >>>> + virCommandSetOutputBuffer(cmd, &output); >>>> + >>>> + if (virCommandRun(cmd, NULL) < 0) >>>> + VIR_FREE(output); >>>> + >>>> + virCommandFree(cmd); >>>> + return output; >>>> +} >>>> + >>>> + >>>> +/** >>>> + * Helper function that receives a listing of file names and >>>> + * calls read_dt_phandle() on each one finding for a match >>>> + * with the given phandle argument. Returns the file name if a >>>> + * match is found, NULL otherwise. >>>> + */ >>>> +static char * >>>> +find_dt_file_with_phandle(char *files, int phandle) >>>> +{ >>>> + char *line, *tmp; >>>> + int ret; >>>> + >>>> + line = strtok_r(files, "\n", &tmp); >>>> + do { >>>> + ret = read_dt_phandle(line); >>>> + if (ret == phandle) >>>> + break; >>>> + } while ((line = strtok_r(NULL, "\n", &tmp)) != NULL); >>>> + >>>> + return line; >>>> +} >>>> + >>>> + >>>> +/** >>>> + * This function receives a string that represents a PCI device, >>>> + * such as '0004:04:00.0', and tells if the device is NVLink2 capable. >>>> + * >>>> + * The logic goes as follows: >>>> + * >>>> + * 1 - get the phandle of a nvlink of the device, reading the 'ibm,npu' >>>> + * attribute; >>>> + * 2 - find the device tree node of the nvlink bus using the phandle >>>> + * found in (1) >>>> + * 3 - get the phandle of the memory region of the nvlink bus >>>> + * 4 - find the device tree node of the memory region using the >>>> + * phandle found in (3) >>>> + * 5 - read the 'reg' value of the memory region. If the value of >>>> + * the second 64 bit value is 0x02 0x00, the device is attached >>>> + * to a NVLink2 bus. >>>> + * >>>> + * If any of these steps fails, the function returns false. >>>> + */ >>>> +static bool >>>> +device_is_nvlink2_capable(const char *device) >>>> +{ >>>> + char *file, *files, *tmp; >>>> + unsigned int *reg; >>>> + int phandle; >>>> + >>>> + if ((virAsprintf(&file, "/sys/bus/pci/devices/%s/of_node/ibm,npu", >>>> + device)) < 0) >>>> + return false; >>>> + >>>> + /* Find phandles of nvlinks: */ >>>> + if ((phandle = read_dt_phandle(file)) == -1) >>>> + return false; >>>> + >>>> + /* Find a DT node for the phandle found */ >>>> + files = retrieve_dt_files_pattern("*device-tree/pci*", "phandle"); >>>> + if (!files) >>>> + return false; >>>> + >>>> + if ((file = find_dt_file_with_phandle(files, phandle)) == NULL) >>>> + goto fail; >>>> + >>>> + /* Find a phandle of the GPU memory region of the device. The >>>> + * file found above ends with '/phandle' - the memory region >>>> + * of the GPU ends with '/memory-region */ >>>> + tmp = strrchr(file, '/'); >>>> + *tmp = '\0'; >>>> + file = strcat(file, "/memory-region"); >>>> + >>>> + if ((phandle = read_dt_phandle(file)) == -1) >>>> + goto fail; >>>> + >>>> + file = NULL; >>>> + VIR_FREE(files); >>>> + >>>> + /* Find the memory node for the phandle found above */ >>>> + files = retrieve_dt_files_pattern("*device-tree/memory*", "phandle"); >>>> + if (!files) >>>> + return false; >>>> + >>>> + if ((file = find_dt_file_with_phandle(files, phandle)) == NULL) >>>> + goto fail; >>>> + >>>> + /* And see its size in the second 64bit value of 'reg'. First, >>>> + * the end of the file needs to be changed from '/phandle' to >>>> + * '/reg' */ >>>> + tmp = strrchr(file, '/'); >>>> + *tmp = '\0'; >>>> + file = strcat(file, "/reg"); >>>> + >>>> + reg = read_dt_memory_reg(file); >>>> + if (reg && reg[2] == 0x20 && reg[3] == 0x00) >>>> + return true; >>> This function is very complex just to find out whether a PCI device is capable >>> of NVLink or not. Feels wrong to do it this way, I believe it would be much >>> easier if NVIDIA exposed a sysfs attribute saying whether a PCI device supports >>> NVLink so that our node-device driver would take care of this during libvirtd >>> startup and then you'd only call a single API from the PPC64 helper you're >>> introducing in the next patch to find out whether you need the alternative >>> formula or not. >>> >>> Honestly, apart from the coding style issues, I don't like this approach and >>> unless there's a really compelling reason for libvirt to do it in a way which >>> involves spawning a 'find' process because of a complex pattern and a bunch of >>> data necessary to filter out, I'd really suggest contacting NVIDIA about this. >>> It's the same for mdevs, NVIDIA exposes a bunch of attributes in sysfs which >>> we're able to read. >> I'll contact NVIDIA and see if there is an easier way (a sysfs attribute, for >> example) and, if doesn't, try to provide a plausibe reason to justify this >> detection code. > Sorry for the delay in responding. The problem is that all the V100 GPUs > support NVLink, but it may or may not be connected up. This is detected > at runtime during GPU initialization, which seems like much too heavy of > an operation to perform as part of passthrough initialization. And that's > why vfio-pci pieces rely on device tree information to figure it out. > > Alexey, would it be possible for vfio-pci to export the information in a > way more friendly to libvirt? The only information needed here is whether a specific GPU has RAM or not. This can easily be found from the device-tree, imho quite friendly already. VFIO gets to know about these new capabilities when the VFIO PCI device is opened, and we rather want to avoid going that far in libvirt (open a VFIO container, attach a group, get a vfio-pci fd from it, enumerate regions - 2 PCI resets on the way, delays, meh). btw the first "find" for "ibm,npu" can be skipped - NVLinks have to be passed too or the entire RAM thing won't work. "find" for the memory node can also be dropped really - if NVLink bridge OF node has "memory-region", then VFIO will most likely expose RAM and QEMU will try using it anyway.

>>> Thinking about it a bit more, since this is NVIDIA-specific, having an >>> NVIDIA-only sysfs attribute doesn't help the node-device driver I mentioned >>> above. In general, we try to avoid introducing vendor-specific code, so nodedev >>> might not be the right place to check for the attribute (because it's not >>> universal and would require vendor-specific elements), but I think we could >>> have an NVLink helper reading this sysfs attribute(s) elsewhere in libvirt >>> codebase. >>> >>> Erik