pc, pc64: do page attribute table (PAT) init early in cpuidentify()

the page attribute table was initialized in mmuinit(), which is
too late for bootscreen(). So now we check for PAT support and
insert the write-combine entry early in cpuidentify().

this might have been the cause of some slow EFI framebuffers on
machines with overlapping or insufficient MTRR entries.

1 files changed, 8 insertions, 1 deletions

diff --git a/sys/src/9/pc/devarch.c b/sys/src/9/pc/devarch.c
index 2da6f5d55..595cbcb8b 100644
--- a/sys/src/9/pc/devarch.c
+++ b/sys/src/9/pc/devarch.c
@@ -749,7 +749,7 @@ cpuidentify(void)
 	X86type *t, *tab;
 	uintptr cr4;
 	ulong regs[4];
-	vlong mca, mct;
+	vlong mca, mct, pat;
 
 	cpuid(Highstdfunc, regs);
 	memmove(m->cpuidid,   &regs[1], BY2WD);	/* bx */
@@ -882,6 +882,13 @@ cpuidentify(void)
 			rdmsr(0x01, &mct);
 	}
 
+	/* IA32_PAT write combining */
+	if((m->cpuiddx & Pat) != 0 && rdmsr(0x277, &pat) != -1){
+		pat &= ~(255LL<<(PATWC*8));
+		pat |= 1LL<<(PATWC*8);	/* WC */
+		wrmsr(0x277, pat);
+	}
+
 	if(m->cpuiddx & Mtrr)
 		mtrrsync();