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

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

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.

> +
> +	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?

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.

Regards,

Tvrtko

> +
> +	pthread_join(thread, NULL);
> +
> +	munmap(t.page, 4096);
> +	close(ufd);
> +}
> +
>   uint64_t total_ram;
>   uint64_t aperture_size;
>   int fd, count;
> @@ -1902,6 +2016,9 @@ igt_main_args("c:", NULL, help_str, opt_handler, NULL)
>   		igt_subtest("forbidden-operations")
>   			test_forbidden_ops(fd);
>   
> +		igt_subtest("userfault")
> +			test_userfault(fd);
> +
>   		igt_subtest("relocations")
>   			test_relocations(fd);
>   	}
> 
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