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

[Hans Petter Selasky]

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.

--HPS