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 18:07, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-11-11 17:54:27)
>>
>> 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).
> 
> That could only be if the fault never occurred, in which case we would
> still be blocking on the read(). I think that's a reasonable level for
> us not to care about -- it's the same as our depending on write()/read()
> for synchronising between parent & child elsewhere. No?

Using userfaultfd is a bit more complex so I thought of extra checks. 
But okay, if probably is overly paranoid.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko
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