[PATCH] swiotlb=nslabs not honored when launching xen pv guests.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

Hey Tomo-san,

I ran into an issue where the 'swiotlb' argument was being ignored when
launching a PV guest. Please see the patch - it has a nice explanation
in the description of the patch.

Not sure if this is the best way of fixing it. The other way of fixing
this would be to change swiotlb_init_with_tbl to sample the 'io_tlb_nslabs'
and if the io_tlb_nslabs is set use that instead of the passed in value?
But that looks a bit sneaky..

Something like this (not tested, nor compiled)?

diff --git a/lib/swiotlb.c b/lib/swiotlb.c
index 93ca08b..3cfad6e 100644
--- a/lib/swiotlb.c
+++ b/lib/swiotlb.c
@@ -136,9 +136,11 @@ void __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose)
 {
 	unsigned long i, bytes;
 
+	if (io_tlb_nslabs)
+		nslabs = io_tlb_nslabs;
+
 	bytes = nslabs << IO_TLB_SHIFT;
 
-	io_tlb_nslabs = nslabs;
 	io_tlb_start = tlb;
 	io_tlb_end = io_tlb_start + bytes;