Linux Virtualization - Mar 2019 - [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()

From: "Michael S. Tsirkin" <mst at redhat.com>
Date: Mon, 11 Mar 2019 09:59:28 -0400
> On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
>> 
>> On 2019/3/8 ??10:12, Christoph Hellwig wrote:
>> > On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:
>> > > This series tries to access virtqueue metadata through kernel
virtual
>> > > address instead of copy_user() friends since they had too
much
>> > > overheads like checks, spec barriers or even hardware feature
>> > > toggling. This is done through setup kernel address through
vmap() and
>> > > resigter MMU notifier for invalidation.
>> > > 
>> > > Test shows about 24% improvement on TX PPS. TCP_STREAM
doesn't see
>> > > obvious improvement.
>> > How is this going to work for CPUs with virtually tagged caches?
>> 
>> 
>> Anything different that you worry?
> 
> If caches have virtual tags then kernel and userspace view of memory
> might not be automatically in sync if they access memory
> through different virtual addresses. You need to do things like
> flush_cache_page, probably multiple times.
"flush_dcache_page()"

On 2019/3/12 ??2:14, David Miller wrote:
> From: "Michael S. Tsirkin" <mst at redhat.com>
> Date: Mon, 11 Mar 2019 09:59:28 -0400
>
>> On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
>>> On 2019/3/8 ??10:12, Christoph Hellwig wrote:
>>>> On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang wrote:
>>>>> This series tries to access virtqueue metadata through
kernel virtual
>>>>> address instead of copy_user() friends since they had too
much
>>>>> overheads like checks, spec barriers or even hardware
feature
>>>>> toggling. This is done through setup kernel address through
vmap() and
>>>>> resigter MMU notifier for invalidation.
>>>>>
>>>>> Test shows about 24% improvement on TX PPS. TCP_STREAM
doesn't see
>>>>> obvious improvement.
>>>> How is this going to work for CPUs with virtually tagged
caches?
>>>
>>> Anything different that you worry?
>> If caches have virtual tags then kernel and userspace view of memory
>> might not be automatically in sync if they access memory
>> through different virtual addresses. You need to do things like
>> flush_cache_page, probably multiple times.
> "flush_dcache_page()"

I get this. Then I think the current set_bit_to_user() is suspicious, we 
probably miss a flush_dcache_page() there:


static int set_bit_to_user(int nr, void __user *addr)
{
 ??????? unsigned long log = (unsigned long)addr;
 ??????? struct page *page;
 ??????? void *base;
 ??????? int bit = nr + (log % PAGE_SIZE) * 8;
 ??????? int r;

 ??????? r = get_user_pages_fast(log, 1, 1, &page);
 ??????? if (r < 0)
 ??????????????? return r;
 ??????? BUG_ON(r != 1);
 ??????? base = kmap_atomic(page);
 ??????? set_bit(bit, base);
 ??????? kunmap_atomic(base);
 ??????? set_page_dirty_lock(page);
 ??????? put_page(page);
 ??????? return 0;
}

Thanks

On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang
wrote:
> 
> On 2019/3/12 ??2:14, David Miller wrote:
> > From: "Michael S. Tsirkin" <mst at redhat.com>
> > Date: Mon, 11 Mar 2019 09:59:28 -0400
> > 
> > > On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
> > > > On 2019/3/8 ??10:12, Christoph Hellwig wrote:
> > > > > On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang
wrote:
> > > > > > This series tries to access virtqueue metadata
through kernel virtual
> > > > > > address instead of copy_user() friends since they
had too much
> > > > > > overheads like checks, spec barriers or even
hardware feature
> > > > > > toggling. This is done through setup kernel
address through vmap() and
> > > > > > resigter MMU notifier for invalidation.
> > > > > > 
> > > > > > Test shows about 24% improvement on TX PPS.
TCP_STREAM doesn't see
> > > > > > obvious improvement.
> > > > > How is this going to work for CPUs with virtually
tagged caches?
> > > > 
> > > > Anything different that you worry?
> > > If caches have virtual tags then kernel and userspace view of
memory
> > > might not be automatically in sync if they access memory
> > > through different virtual addresses. You need to do things like
> > > flush_cache_page, probably multiple times.
> > "flush_dcache_page()"
> 
> 
> I get this. Then I think the current set_bit_to_user() is suspicious, we
> probably miss a flush_dcache_page() there:
> 
> 
> static int set_bit_to_user(int nr, void __user *addr)
> {
> ??????? unsigned long log = (unsigned long)addr;
> ??????? struct page *page;
> ??????? void *base;
> ??????? int bit = nr + (log % PAGE_SIZE) * 8;
> ??????? int r;
> 
> ??????? r = get_user_pages_fast(log, 1, 1, &page);
> ??????? if (r < 0)
> ??????????????? return r;
> ??????? BUG_ON(r != 1);
> ??????? base = kmap_atomic(page);
> ??????? set_bit(bit, base);
> ??????? kunmap_atomic(base);
> ??????? set_page_dirty_lock(page);
> ??????? put_page(page);
> ??????? return 0;
> }
> 
> Thanks
I think you are right. The correct fix though is to re-implement
it using asm and handling pagefault, not gup.
Three atomic ops per bit is way to expensive.

-- 
MST

On 2019/3/12 ??1:14, James Bottomley wrote:
> On Tue, 2019-03-12 at 10:59 +0800, Jason Wang wrote:
>> On 2019/3/12 ??2:14, David Miller wrote:
>>> From: "Michael S. Tsirkin" <mst at redhat.com>
>>> Date: Mon, 11 Mar 2019 09:59:28 -0400
>>>
>>>> On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
>>>>> On 2019/3/8 ??10:12, Christoph Hellwig wrote:
>>>>>> On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason Wang
wrote:
>>>>>>> This series tries to access virtqueue metadata
through
>>>>>>> kernel virtual
>>>>>>> address instead of copy_user() friends since they
had too
>>>>>>> much
>>>>>>> overheads like checks, spec barriers or even
hardware
>>>>>>> feature
>>>>>>> toggling. This is done through setup kernel address
through
>>>>>>> vmap() and
>>>>>>> resigter MMU notifier for invalidation.
>>>>>>>
>>>>>>> Test shows about 24% improvement on TX PPS.
TCP_STREAM
>>>>>>> doesn't see
>>>>>>> obvious improvement.
>>>>>> How is this going to work for CPUs with virtually
tagged
>>>>>> caches?
>>>>> Anything different that you worry?
>>>> If caches have virtual tags then kernel and userspace view of
>>>> memory
>>>> might not be automatically in sync if they access memory
>>>> through different virtual addresses. You need to do things like
>>>> flush_cache_page, probably multiple times.
>>> "flush_dcache_page()"
>>
>> I get this. Then I think the current set_bit_to_user() is suspicious,
>> we
>> probably miss a flush_dcache_page() there:
>>
>>
>> static int set_bit_to_user(int nr, void __user *addr)
>> {
>>           unsigned long log = (unsigned long)addr;
>>           struct page *page;
>>           void *base;
>>           int bit = nr + (log % PAGE_SIZE) * 8;
>>           int r;
>>
>>           r = get_user_pages_fast(log, 1, 1, &page);
>>           if (r < 0)
>>                   return r;
>>           BUG_ON(r != 1);
>>           base = kmap_atomic(page);
>>           set_bit(bit, base);
>>           kunmap_atomic(base);
> This sequence should be OK.  get_user_pages() contains a flush which
> clears the cache above the user virtual address, so on kmap, the page
> is coherent at the new alias.  On parisc at least, kunmap embodies a
> flush_dcache_page() which pushes any changes in the cache above the
> kernel virtual address back to main memory and makes it coherent again
> for the user alias to pick it up.

It would be good if kmap()/kunmap() can do this but looks like we can 
not assume this? For example, sparc's flush_dcache_page() doesn't do 
flush_dcache_page(). And bio_copy_data_iter() do flush_dcache_page() 
after kunmap_atomic().

Thanks

>
> James
>

On 2019/3/12 ??3:51, Jason Wang wrote:
>
> On 2019/3/12 ??1:14, James Bottomley wrote:
>> On Tue, 2019-03-12 at 10:59 +0800, Jason Wang wrote:
>>> On 2019/3/12 ??2:14, David Miller wrote:
>>>> From: "Michael S. Tsirkin" <mst at redhat.com>
>>>> Date: Mon, 11 Mar 2019 09:59:28 -0400
>>>>
>>>>> On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
>>>>>> On 2019/3/8 ??10:12, Christoph Hellwig wrote:
>>>>>>> On Wed, Mar 06, 2019 at 02:18:07AM -0500, Jason
Wang wrote:
>>>>>>>> This series tries to access virtqueue metadata
through
>>>>>>>> kernel virtual
>>>>>>>> address instead of copy_user() friends since
they had too
>>>>>>>> much
>>>>>>>> overheads like checks, spec barriers or even
hardware
>>>>>>>> feature
>>>>>>>> toggling. This is done through setup kernel
address through
>>>>>>>> vmap() and
>>>>>>>> resigter MMU notifier for invalidation.
>>>>>>>>
>>>>>>>> Test shows about 24% improvement on TX PPS.
TCP_STREAM
>>>>>>>> doesn't see
>>>>>>>> obvious improvement.
>>>>>>> How is this going to work for CPUs with virtually
tagged
>>>>>>> caches?
>>>>>> Anything different that you worry?
>>>>> If caches have virtual tags then kernel and userspace view
of
>>>>> memory
>>>>> might not be automatically in sync if they access memory
>>>>> through different virtual addresses. You need to do things
like
>>>>> flush_cache_page, probably multiple times.
>>>> "flush_dcache_page()"
>>>
>>> I get this. Then I think the current set_bit_to_user() is
suspicious,
>>> we
>>> probably miss a flush_dcache_page() there:
>>>
>>>
>>> static int set_bit_to_user(int nr, void __user *addr)
>>> {
>>> ????????? unsigned long log = (unsigned long)addr;
>>> ????????? struct page *page;
>>> ????????? void *base;
>>> ????????? int bit = nr + (log % PAGE_SIZE) * 8;
>>> ????????? int r;
>>>
>>> ????????? r = get_user_pages_fast(log, 1, 1, &page);
>>> ????????? if (r < 0)
>>> ????????????????? return r;
>>> ????????? BUG_ON(r != 1);
>>> ????????? base = kmap_atomic(page);
>>> ????????? set_bit(bit, base);
>>> ????????? kunmap_atomic(base);
>> This sequence should be OK.? get_user_pages() contains a flush which
>> clears the cache above the user virtual address, so on kmap, the page
>> is coherent at the new alias.? On parisc at least, kunmap embodies a
>> flush_dcache_page() which pushes any changes in the cache above the
>> kernel virtual address back to main memory and makes it coherent again
>> for the user alias to pick it up.
>
>
> It would be good if kmap()/kunmap() can do this but looks like we can 
> not assume this? For example, sparc's flush_dcache_page() 

Sorry, I meant kunmap_atomic().

Thanks

> doesn't do flush_dcache_page(). And bio_copy_data_iter() do 
> flush_dcache_page() after kunmap_atomic().
>
> Thanks
>
>
>>
>> James
>>
>