Re: [virtio] [PATCH RFC] VIRTIO_F_PARTIAL_ORDER for page fault handling

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 11 Aug 2020 04:23:13 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Aug 10, 2020 at 06:59:28PM +0200, Cornelia Huck wrote:
> > On Mon, 10 Aug 2020 12:15:15 -0400
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:

> > > diff --git a/split-ring.tex b/split-ring.tex
> > > index 123ac9f..cf197f8 100644
> > > --- a/split-ring.tex
> > > +++ b/split-ring.tex
> > > @@ -398,10 +398,11 @@ \subsection{The Virtqueue Used Ring}\label{sec:Basic Facilities of a Virtio Devi
> > >          le16 avail_event; /* Only if VIRTIO_F_EVENT_IDX */
> > >  };
> > >  
> > > -/* le32 is used here for ids for padding reasons. */
> > >  struct virtq_used_elem {
> > >          /* Index of start of used descriptor chain. */
> > > -        le32 id;
> > > +        le16 id;
> > > +#define VIRTQ_USED_ELEM_F_FLUSH  0x8000
> > > +        le16 flags;  
> > 
> > This is a bit confusing for legacy, as this will only work for
> > little-endian. The flags field will only be used for non-legacy, but I
> > think the id field should still be treated as 32 bit for legacy.
> > (We have a wholesale "it's guest endian when legacy" further up.)  
> 
> True. How do you suggest handling this? Just say that id is
> OR with VIRTQ_USED_ELEM_F_FLUSH ? Or maybe just add
> struct virtq_used_elem_legacy?

I think introducing a legacy struct is the least confusing option.

> 
> 
> > >          /* Total length of the descriptor chain which was used (written to) */
> > >          le32 len;
> > >  };


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