Re: [igt-dev] [PATCH i-g-t 2/3] i915/gem_userptr_blits: Exercise userptr + userfaultfd

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 11/11/2019 16:58, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-11-11 16:48:14)
>>
>> On 08/11/2019 20:49, Chris Wilson wrote:
>>> Register a userspace fault handler for a memory region that we also pass
>>> to the GPU via userptr, and make sure the pagefault is properly serviced
>>> before we execute.
>>>
>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>> ---
>>>    tests/i915/gem_userptr_blits.c | 119 ++++++++++++++++++++++++++++++++-
>>>    1 file changed, 118 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tests/i915/gem_userptr_blits.c b/tests/i915/gem_userptr_blits.c
>>> index 11d6f4a1c..774a9f92c 100644
>>> --- a/tests/i915/gem_userptr_blits.c
>>> +++ b/tests/i915/gem_userptr_blits.c
>>> @@ -36,6 +36,8 @@
>>>     * The goal is to simply ensure the basics work.
>>>     */
>>>    
>>> +#include <linux/userfaultfd.h>
>>> +
>>>    #include "igt.h"
>>>    #include <stdlib.h>
>>>    #include <stdio.h>
>>> @@ -44,9 +46,11 @@
>>>    #include <inttypes.h>
>>>    #include <errno.h>
>>>    #include <setjmp.h>
>>> +#include <sys/ioctl.h>
>>> +#include <sys/mman.h>
>>>    #include <sys/stat.h>
>>> +#include <sys/syscall.h>
>>>    #include <sys/time.h>
>>> -#include <sys/mman.h>
>>>    #include <glib.h>
>>>    #include <signal.h>
>>>    #include <pthread.h>
>>> @@ -1831,6 +1835,116 @@ static void test_invalidate_close_race(int fd, bool overlap)
>>>        free(t_data.ptr);
>>>    }
>>>    
>>> +struct ufd_thread {
>>> +     uint32_t *page;
>>> +     int i915;
>>> +};
>>> +
>>> +static uint32_t create_page(int i915, void *page)
>>> +{
>>> +     uint32_t handle;
>>> +
>>> +     gem_userptr(i915, page, 4096, 0, 0, &handle);
>>> +     return handle;
>>> +}
>>> +
>>> +static uint32_t create_batch(int i915)
>>> +{
>>> +     const uint32_t bbe = MI_BATCH_BUFFER_END;
>>> +     uint32_t handle;
>>> +
>>> +     handle = gem_create(i915, 4096);
>>> +     gem_write(i915, handle, 0, &bbe, sizeof(bbe));
>>> +
>>> +     return handle;
>>> +}
>>> +
>>> +static void *ufd_thread(void *arg)
>>> +{
>>> +     struct ufd_thread *t = arg;
>>> +     struct drm_i915_gem_exec_object2 obj[2] = {
>>> +             { .handle = create_page(t->i915, t->page) },
>>> +             { .handle = create_batch(t->i915) },
>>> +     };
>>> +     struct drm_i915_gem_execbuffer2 eb = {
>>> +             .buffers_ptr = to_user_pointer(obj),
>>> +             .buffer_count = ARRAY_SIZE(obj),
>>> +     };
>>> +
>>> +     igt_debug("submitting fault\n");
>>> +     gem_execbuf(t->i915, &eb);
>>> +     gem_sync(t->i915, obj[1].handle);
>>> +
>>> +     for (int i = 0; i < ARRAY_SIZE(obj); i++)
>>> +             gem_close(t->i915, obj[i].handle);
>>> +
>>> +     t->i915 = -1;
>>> +     return NULL;
>>> +}
>>> +
>>> +static int userfaultfd(int flags)
>>> +{
>>> +     return syscall(SYS_userfaultfd, flags);
>>> +}
>>> +
>>> +static void test_userfault(int i915)
>>> +{
>>> +     struct uffdio_api api = { .api = UFFD_API };
>>> +     struct uffdio_register reg;
>>> +     struct uffdio_copy copy;
>>> +     struct uffd_msg msg;
>>> +     struct ufd_thread t;
>>> +     pthread_t thread;
>>> +     char poison[4096];
>>> +     int ufd;
>>> +
>>> +     /*
>>> +      * Register a page with userfaultfd, and wrap that inside a userptr bo.
>>> +      * When we try to use gup insider userptr_get_pages, it will trigger
>>> +      * a pagefault that is sent to the userfaultfd for servicing. This
>>> +      * is arbitrarily slow, as the submission must wait until the fault
>>> +      * is serviced by the userspace fault handler.
>>> +      */
>>> +
>>> +     ufd = userfaultfd(0);
>>> +     igt_require_f(ufd != -1, "kernel support for userfaultfd\n");
>>> +     igt_require_f(ioctl(ufd, UFFDIO_API, &api) == 0 && api.api == UFFD_API,
>>> +                   "userfaultfd API v%lld:%lld\n", UFFD_API, api.api);
>>> +
>>> +     t.i915 = i915;
>>> +
>>> +     t.page = mmap(NULL, 4096, PROT_WRITE, MAP_SHARED | MAP_ANON, 0, 0);
>>> +     igt_assert(t.page != MAP_FAILED);
>>> +
>>> +     memset(&reg, 0, sizeof(reg));
>>> +     reg.mode = UFFDIO_REGISTER_MODE_MISSING;
>>> +     reg.range.start = to_user_pointer(t.page);
>>> +     reg.range.len = 4096;
>>> +     do_ioctl(ufd, UFFDIO_REGISTER, &reg);
>>> +     igt_assert(reg.ioctls == UFFD_API_RANGE_IOCTLS);
>>> +
>>> +     igt_assert(pthread_create(&thread, NULL, ufd_thread, &t) == 0);
>>> +
>>> +     /* Wait for the fault */
>>> +     igt_assert_eq(read(ufd, &msg, sizeof(msg)), sizeof(msg));
>>> +     igt_assert_eq(msg.event, UFFD_EVENT_PAGEFAULT);
>>> +     igt_assert(from_user_pointer(msg.arg.pagefault.address) == t.page);
>>> +
>>> +     /* Faulting thread remains blocked */
>>> +     igt_assert_eq(t.i915, i915);
>>
>> This looks could be a false negative since nothing says the thread is
>> not blocked just not got round resetting t->i915.
> 
> There's a gem_sync() in the thread. Our goal is that the thread is
> blocked (either at submit or in the sync) until we service the fault.

What I meant was it could have passed gem_execbuf and gem_sync, just not 
got to the t->i915 = -1 line yet. Am I being to pedantic? Maybe using 
output fence and passing it back to parent thread would be easier? 
Parent then does igt_assert_eq(poll(fd, some_timeout), 0).
>>> +     memset(&copy, 0, sizeof(copy));
>>> +     copy.dst = msg.arg.pagefault.address;
>>> +     copy.src = to_user_pointer(memset(poison, 0xc5, sizeof(poison)));
>>> +     copy.len = 4096;
>>> +     do_ioctl(ufd, UFFDIO_COPY, &copy);
>>
>> What is the point of poison data?
> 
> Just a tell tale.
>   
>> Would it work better to have a hanging batch registered with userfault
>> and then replace it with a valid batch here? That would ensure execbuf
>> was  blocked until userfault handler finishes otherwise we get a GPU hang.
> 
> Strictly we can't use userptr for the batch itself, as aiui that
> requires LLC for the CS coherency. One of the following tests uses the
> userfault handler as the sync point for replacing a poisoned batch with
> a valid one, with the intention of looking for the GPU hang if the fault
> was missed. That's the test I started with, my goal here was to focus
> on the userptr + userfault as simply as I could; so gem_sync().

Okay that's fair then.

Regards,

Tvrtko
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