RE: [RFC 03/11] Drivers: hv: vmbus: Introduce types of GPADL

[Michael Kelley <mikelley@microsoft.com>]

From: Boqun Feng <boqun.feng@gmail.com> Sent: Wednesday, July 22, 2020 4:43 PM
> 
> On Wed, Jul 22, 2020 at 11:25:18PM +0000, Michael Kelley wrote:
> > From: Boqun Feng <boqun.feng@gmail.com> Sent: Monday, July 20, 2020 6:41 PM
> > >
> > > This patch introduces two types of GPADL: HV_GPADL_{BUFFER, RING}. The
> > > types of GPADL are purely the concept in the guest, IOW the hypervisor
> > > treat them as the same.
> > >
> > > The reason of introducing the types of GPADL is to support guests whose
> > > page size is not 4k (the page size of Hyper-V hypervisor). In these
> > > guests, both the headers and the data parts of the ringbuffers need to
> > > be aligned to the PAGE_SIZE, because 1) some of the ringbuffers will be
> > > mapped into userspace and 2) we use "double mapping" mechanism to
> > > support fast wrap-around, and "double mapping" relies on ringbuffers
> > > being page-aligned. However, the Hyper-V hypervisor only uses 4k
> > > (HV_HYP_PAGE_SIZE) headers. Our solution to this is that we always make
> > > the headers of ringbuffers take one guest page and when GPADL is
> > > established between the guest and hypervisor, the only first 4k of
> > > header is used. To handle this special case, we need the types of GPADL
> > > to differ different guest memory usage for GPADL.
> > >
> > > Type enum is introduced along with several general interfaces to
> > > describe the differences between normal buffer GPADL and ringbuffer
> > > GPADL.
> > >
> > > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > > ---
> > >  drivers/hv/channel.c   | 140 +++++++++++++++++++++++++++++++++++------
> > >  include/linux/hyperv.h |  44 ++++++++++++-
> > >  2 files changed, 164 insertions(+), 20 deletions(-)
> >
> > [snip]
> >
> > >
> > >
> > > @@ -437,7 +528,17 @@ static int __vmbus_open(struct vmbus_channel *newchannel,
> > >  	open_msg->openid = newchannel->offermsg.child_relid;
> > >  	open_msg->child_relid = newchannel->offermsg.child_relid;
> > >  	open_msg->ringbuffer_gpadlhandle = newchannel->ringbuffer_gpadlhandle;
> > > -	open_msg->downstream_ringbuffer_pageoffset = newchannel-
> > > >ringbuffer_send_offset;
> > > +	/*
> > > +	 * The unit of ->downstream_ringbuffer_pageoffset is HV_HYP_PAGE and
> > > +	 * the unit of ->ringbuffer_send_offset is PAGE, so here we first
> > > +	 * calculate it into bytes and then convert into HV_HYP_PAGE. Also
> > > +	 * ->ringbuffer_send_offset is the offset in guest, while
> > > +	 * ->downstream_ringbuffer_pageoffset is the offset in gpadl (i.e. in
> > > +	 * hypervisor), so a (PAGE_SIZE - HV_HYP_PAGE_SIZE) gap need to be
> > > +	 * skipped.
> > > +	 */
> > > +	open_msg->downstream_ringbuffer_pageoffset =
> > > +		((newchannel->ringbuffer_send_offset << PAGE_SHIFT) - (PAGE_SIZE -
> > > HV_HYP_PAGE_SIZE)) >> HV_HYP_PAGE_SHIFT;
> >
> > I couldn't find that the "downstream_ringbuffer_pageoffset" field
> > is used anywhere.  Can it just be deleted entirely instead of having
> > this really messy calculation?
> >
> 
> This field is part of struct vmbus_channel_open_channel, which means
> guest-hypervisor communication protocal requires us to set the field,
> IIUC. So I don't think we can delete it.

Indeed, you are right.  I mis-read it as a field in struct vmbus_channel,
but that's not the case.  Thanks.

> 
> To deal with the messy calculation, I do realize there is a similar
> calculation in hv_gpadl_hvpfn() too, so in the next version, I will
> add a new helper to do this "send offset in guest virtual address to
> send offset in GPADL calculation", and use it here and in
> hv_gpadl_hvpfn(). Thoughts?

Yes, that helps.

> 
> > >  	open_msg->target_vp = newchannel->target_vp;
> > >
> > >  	if (userdatalen)
> > > @@ -497,6 +598,7 @@ static int __vmbus_open(struct vmbus_channel *newchannel,
> > >  	return err;
> > >  }
> > >
> > > +
> >
> > Spurious add of a blank line?
> >
> 
> Yeah, I will fix this, thanks!
> 
> Regards,
> Boqun
> 
> > >  /*
> > >   * vmbus_connect_ring - Open the channel but reuse ring buffer
> > >   */
> > > diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> > > index 692c89ccf5df..663f0a016237 100644
> > > --- a/include/linux/hyperv.h
> > > +++ b/include/linux/hyperv.h
> > > @@ -29,6 +29,48 @@
> > >
> > >  #pragma pack(push, 1)
> > >
> > > +/*
> > > + * Types for GPADL, decides is how GPADL header is created.
> > > + *
> > > + * It doesn't make much difference between BUFFER and RING if PAGE_SIZE is the
> > > + * same as HV_HYP_PAGE_SIZE.
> > > + *
> > > + * If PAGE_SIZE is bigger than HV_HYP_PAGE_SIZE, the headers of ring buffers
> > > + * will be of PAGE_SIZE, however, only the first HV_HYP_PAGE will be put
> > > + * into gpadl, therefore the number for HV_HYP_PAGE and the indexes of each
> > > + * HV_HYP_PAGE will be different between different types of GPADL, for example
> > > + * if PAGE_SIZE is 64K:
> > > + *
> > > + * BUFFER:
> > > + *
> > > + * gva:    |--       64k      --|--       64k      --| ... |
> > > + * gpa:    | 4k | 4k | ... | 4k | 4k | 4k | ... | 4k |
> > > + * index:  0    1    2     15   16   17   18 .. 31   32 ...
> > > + *         |    |    ...   |    |    |   ...    |   ...
> > > + *         v    V          V    V    V          V
> > > + * gpadl:  | 4k | 4k | ... | 4k | 4k | 4k | ... | 4k | ... |
> > > + * index:  0    1    2 ... 15   16   17   18 .. 31   32 ...
> > > + *
> > > + * RING:
> > > + *
> > > + *         | header  |           data           | header  |     data      |
> > > + * gva:    |-- 64k --|--       64k      --| ... |-- 64k --|-- 64k --| ... |
> > > + * gpa:    | 4k | .. | 4k | 4k | ... | 4k | ... | 4k | .. | 4k | .. | ... |
> > > + * index:  0    1    16   17   18    31   ...   n   n+1  n+16 ...         2n
> > > + *         |         /    /          /          |         /               /
> > > + *         |        /    /          /           |        /               /
> > > + *         |       /    /   ...    /    ...     |       /      ...      /
> > > + *         |      /    /          /             |      /               /
> > > + *         |     /    /          /              |     /               /
> > > + *         V    V    V          V               V    V               v
> > > + * gpadl:  | 4k | 4k |   ...    |    ...        | 4k | 4k |  ...     |
> > > + * index:  0    1    2   ...    16   ...       n-15 n-14 n-13  ...  2n-30
> > > + */
> > > +enum hv_gpadl_type {
> > > +	HV_GPADL_BUFFER,
> > > +	HV_GPADL_RING
> > > +};
> > > +
> > >  /* Single-page buffer */
> > >  struct hv_page_buffer {
> > >  	u32 len;
> > > @@ -111,7 +153,7 @@ struct hv_ring_buffer {
> > >  	} feature_bits;
> > >
> > >  	/* Pad it to PAGE_SIZE so that data starts on page boundary */
> > > -	u8	reserved2[4028];
> > > +	u8	reserved2[PAGE_SIZE - 68];
> > >
> > >  	/*
> > >  	 * Ring data starts here + RingDataStartOffset
> > > --
> > > 2.27.0
> >