Re: [PATCH net] vhost: flush dcache page when logging dirty pages

[Jason Wang]

On 2019/4/9 äå9:14, Michael S. Tsirkin wrote:
> On Tue, Apr 09, 2019 at 12:16:47PM +0800, Jason Wang wrote:
> > We set dirty bit through setting up kmaps and access them through
> > kernel virtual address, this may result alias in virtually tagged
> > caches that require a dcache flush afterwards.
> >
> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> > Cc: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > Fixes: 3a4d5c94e9593 ("vhost_net: a kernel-level virtio server")
> This is like saying "everyone with vhost needs this".
> In practice only might affect some architectures.


For the archs that does need dcache flushing, the function is just a nop.


> Which ones?


There're more than 10 archs that have ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE 
defined, just cc some maintainers of some more influenced ones.


> You want to Cc the relevant maintainers
> who understand this...
>
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> I am not sure this is a good idea.
> The region in question is supposed to be accessed
> by userspace at the same time, through atomic operations.
>
> How do we know userspace didn't access it just before?


get_user_pages() will do both flush_annon_page() to make sure the 
userspace write is visible to kernel.


> Is that an issue at all given we use
> atomics for access? Documentation/core-api/cachetlb.rst does
> not mention atomics.
> Which architectures are affected?
> Assuming atomics actually do need a flush, then don't we need
> a flush in the other direction too? How are atomics
> supposed to work at all?


It's the issue of visibility, atomic operation is just one of the 
possible operations. If we can finally makes the write visible to each 
other, there will be no issue.

It looks to me we could still end up alias if userspace is accessing the 
dirty log between get_user_pages_fast() and flush_dcache_page(). But the 
flush_dcache_page() can guarantee what kernel wrote is visible to 
userspace finally though some bits cleared by userspace might still 
there. We may end up with more dirty pages noticed by userspace which 
should be harmless.


> I really think we need new APIs along the lines of
> set_bit_to_user.


Can we simply do:

get_user()

set bit

put_user()

instead?


> > ---
> >   drivers/vhost/vhost.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 351af88231ad..34a1cedbc5ba 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -1711,6 +1711,7 @@ static int set_bit_to_user(int nr, void __user *addr)
> >   	base = kmap_atomic(page);
> >   	set_bit(bit, base);
> >   	kunmap_atomic(base);
> > +	flush_dcache_page(page);
> >   	set_page_dirty_lock(page);
> >   	put_page(page);
> >   	return 0;
> Ignoring the question of whether this actually helps, I doubt
> flush_dcache_page is appropriate here.  Pls take a look at
> Documentation/core-api/cachetlb.rst as well as the actual
> implementation.
>
> I think you meant flush_kernel_dcache_page, and IIUC it must happen
> before kunmap, not after (which you still have the va locked).


Looks like you're right.

Thanks


> > --
> > 2.19.1