Re: [PATCH 05/15] Add io_uring IO interface

On 1/17/19 7:34 AM, Roman Penyaev wrote:

On 2019-01-17 14:54, Jens Axboe wrote:

quoted

On 1/17/19 5:02 AM, Roman Penyaev wrote:

quoted

Hi Jens,

On 2019-01-16 18:49, Jens Axboe wrote:

[...]

quoted

+static void *io_mem_alloc(size_t size)
+{
+	gfp_t gfp_flags = GFP_KERNEL | __GFP_ZERO | __GFP_NOWARN | 
__GFP_COMP
|
+				__GFP_NORETRY;
+
+	return (void *) __get_free_pages(gfp_flags, get_order(size));

Since these pages are shared between kernel and userspace, do we need
to care about d-cache aliasing on armv6 (or other "strange" archs
which I've never seen) with vivt or vipt cpu caches?

E.g. vmalloc_user() targets this problem by aligning kernel address
on SHMLBA, so no flush_dcache_page() is required.

I'm honestly not sure, it'd be trivial enough to stick a
flush_dcache_page() into the few areas we'd need it. The rings are
already page (SHMLBA) aligned.

For arm SHMLBA is not a page, it is 4x page.  So for userspace vaddr
which mmap() returns is aligned, but for kernel not.  So indeed
flush_dcache_page() should be used.

Oh indeed, my bad.

The other question which I can't answer myself is the order of
flush_dcache_page() and smp_wmb().  Does flush_scache_page() implies
flush of the cpu write buffer?   Or firstly smp_wmb() should be done
in order to flush everything to cache.  Here is what arm spec says
about write-back cache:

"Writes that miss in the cache are placed in the write buffer and
appear on the AMBA ASB interface. The CPU continues execution as
soon as the write is placed in the write buffer."

So if you firstly do flush_dcache_page() will it flush write buffer?
Because it seems that firstly smp_wmb() and then flush_dcache_page(),
or I am going mad?

I don't think you're going mad! We'd first need smp_wmb() to order the
writes, then the flush_dcache_page(). For filling the CQ ring, we'd also
need to flush the page the cqe belongs to.

Question is if we care enough about performance on vivt to do something
about that. I know what my answer will be... If others care, they can
incrementally improve upon that.

-- 
Jens Axboe