From cef91f23e787a9e01916b35cf35347b964f03a3b Mon Sep 17 00:00:00 2001
From: cinap_lenrek <cinap_lenrek@felloff.net>
Date: Thu, 10 Dec 2020 01:47:19 +0100
Subject: [PATCH] pc, pc64: work around bhyve all uncached MTRR's

---
 sys/src/9/pc/memory.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/sys/src/9/pc/memory.c b/sys/src/9/pc/memory.c
index 6df499728..f15f6e776 100644
--- a/sys/src/9/pc/memory.c
+++ b/sys/src/9/pc/memory.c
@@ -383,8 +383,16 @@ e820scan(void)
 		}
 	}
 
-	/* RAM needs to be writeback */
-	mtrrexclude(MemRAM, "wb");
+	/*
+	 * Make sure RAM is set to writeback,
+	 * but do a sanity check first checking
+	 * that the kernel text is writeback.
+	 * This is needed as some emulators (bhyve)
+	 * set everything to uncached.
+	 */
+	s = mtrrattr(PADDR(KTZERO), nil);
+	if(s != nil && strcmp(s, "wb") == 0)
+		mtrrexclude(MemRAM, "wb");
 
 	for(base = memmapnext(-1, MemRAM); base != -1; base = memmapnext(base, MemRAM)){
 		size = memmapsize(base, BY2PG) & ~(BY2PG-1);