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pc, pc64: work around bhyve all uncached MTRR's

--- a/sys/src/9/pc/memory.c

+++ b/sys/src/9/pc/memory.c

@@ -383,8 +383,16 @@

 		}

 	}

-	/* 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);

-- 

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