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
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