mbuf: Only allow extpg mbufs if the system has a direct map

Some upcoming changes will modify software checksum routines like
in_cksum() to operate using m_apply(), which uses the direct map to
access packet data for unmapped mbufs.  This approach of course does not
work on platforms without a direct map, so we have to disallow the use
of unmapped mbufs on such platforms.

I believe this is the right tradeoff: we only configure KTLS on amd64
and arm64 today (and one KTLS consumer, NFS TLS, requires a direct map
already), and the use of unmapped mbufs with plain sendfile is a recent
optimization.  If need be, m_apply() could be modified to create
CPU-private mappings of extpg mbuf pages as a fallback.

So, change mb_use_ext_pgs to be hard-wired to zero on systems without a
direct map.  Note that PMAP_HAS_DMAP is not a compile-time constant on
some systems, so the default value of mb_use_ext_pgs has to be
determined during boot.

Reviewed by:	jhb
Discussed with:	gallatin
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D32940

1 files changed, 1 insertions, 1 deletions

diff --git a/sys/rpc/rpcsec_tls/rpctls_impl.c b/sys/rpc/rpcsec_tls/rpctls_impl.c
index 110ba107540a..c495213b08e2 100644
--- a/sys/rpc/rpcsec_tls/rpctls_impl.c
+++ b/sys/rpc/rpcsec_tls/rpctls_impl.c
@@ -711,7 +711,7 @@ rpctls_getinfo(u_int *maxlenp, bool rpctlscd_run, bool rpctlssd_run)
 	int error;
 	size_t siz;
 
-	if (PMAP_HAS_DMAP == 0 || !mb_use_ext_pgs)
+	if (!mb_use_ext_pgs)
 		return (false);
 	siz = sizeof(enable);
 	error = kernel_sysctlbyname(curthread, "kern.ipc.tls.enable",