Re: easy way to work around a lack of a direct map on i386

On Fri, Jan 31, 2020 at 10:13:58AM +0100, Hans Petter Selasky wrote:
> On 2020-01-31 00:37, Konstantin Belousov wrote:
> > On Thu, Jan 30, 2020 at 11:23:02PM +0000, Rick Macklem wrote:
> > > Hi,
> > > 
> > > The current code for KERN_TLS uses PHYS_TO_DMAP()
> > > to access unmapped external pages on m_ext.ext_pgs
> > > mbufs.
> > > I also need to do this to implement RPC-over-TLS.
> > > 
> > > The problem is that some arches, like i386, don't
> > > support PHYS_TO_DMAP().
> > > 
> > > Since it appears that there will be at most 4 pages on
> > > one of these mbufs, my thinking was...
> > > - Acquire four pages of kva from the kernel_map during
> > >    booting.
> > > - Then just use pmap_qenter() to fill in the physical page
> > >    mappings for long enough to copy the data.
> > > 
> > > Does this sound reasonable?
> > > Is there a better way?
> > 
> > Use sfbufs, they should work on all arches.  In essence, they provide MI
> > interface to DMAP where possible.  I do not remember did I bumped the
> > limit for i386 after 4/4 went in.
> > 
> > There is currently no limits for sfbufs use per subsystem, but I think it
> > is not very likely to cause too much troubles.  Main rule is to not sleep
> > waiting for more sfbufs if you already own one..
> 
> In the DRM-KMS LinuxKPI we have:
> 
> void *
> kmap(vm_page_t page)
> {
> #ifdef LINUXKPI_HAVE_DMAP
>         vm_offset_t daddr;
> 
>         daddr = PHYS_TO_DMAP(VM_PAGE_TO_PHYS(page));
> 
>         return ((void *)daddr);
> #else
>         struct sf_buf *sf;
> 
>         sched_pin();
>         sf = sf_buf_alloc(page, SFB_NOWAIT | SFB_CPUPRIVATE);
>         if (sf == NULL) {
>                 sched_unpin();
>                 return (NULL);
>         }
>         return ((void *)sf_buf_kva(sf));
> #endif
> }
> 
> void
> kunmap(vm_page_t page)
> {
> #ifdef LINUXKPI_HAVE_DMAP
>         /* NOP */
> #else
>         struct sf_buf *sf;
> 
>         /* lookup SF buffer in list */
>         sf = sf_buf_alloc(page, SFB_NOWAIT | SFB_CPUPRIVATE);
> 
>         /* double-free */
>         sf_buf_free(sf);
>         sf_buf_free(sf);
> 
>         sched_unpin();
> #endif
> }
> 
> I think that is the fastest way to do this.

So the kmap address is only valid on the CPU that called the function ?
This is strange, I was not able to find mention of it in references to
kmap.