pc64: implement simple write combining for framebuffers with the PAT

on some modern machines like the x250, the bios arranges the mtrr's
and the framebuffer membar in a way that doesnt allow us to mark
the framebuffer pages as write combining, leading to slow graphics.

since the pentium III, the processor interprets the page table bit
combinations of the WT, CD and bit7 bits as an index into the
page attribute table (PAT).

to not change the semantics of the WT and CD bits, we preserve
the bit patterns 0-3 and use the last entry 7 for write combining.
(done in mmuinit() for each core).

the new patwc() function takes virtual address range and changes
the page table marking the range as write combining. no attempt
is made on invalidating tlb's. doesnt matter in our case as the
following mtrr() call in screen.c does it for us.

2 files changed, 36 insertions, 1 deletions

diff --git a/sys/src/9/pc64/fns.h b/sys/src/9/pc64/fns.h
index 4b55e7399..41e3380ec 100644
--- a/sys/src/9/pc64/fns.h
+++ b/sys/src/9/pc64/fns.h
@@ -117,6 +117,7 @@ void	outss(int, void*, int);
 void	outl(int, ulong);
 void	outsl(int, void*, int);
 uintptr	paddr(void*);
+void	patwc(void*, int);
 ulong	pcibarsize(Pcidev*, int);
 void	pcibussize(Pcidev*, ulong*, ulong*);
 int	pcicfgr8(Pcidev*, int);
diff --git a/sys/src/9/pc64/mmu.c b/sys/src/9/pc64/mmu.c
index 18b8878f7..ffd077e31 100644
--- a/sys/src/9/pc64/mmu.c
+++ b/sys/src/9/pc64/mmu.c
@@ -31,13 +31,16 @@ static struct {
 	ulong	nfree;
 } mmupool;
 
-/* level */
 enum {
+	/* level */
 	PML4E	= 2,
 	PDPE	= 1,
 	PDE	= 0,
 
 	MAPBITS	= 8*sizeof(m->mmumap[0]),
+
+	/* PAT entry used for write combining */
+	PATWC	= 7,
 };
 
 static void
@@ -130,6 +133,12 @@ mmuinit(void)
 
 	/* SYSCALL flags mask */
 	wrmsr(0xc0000084, 0x200);
+
+	/* IA32_PAT write combining */
+	rdmsr(0x277, &v);
+	v &= ~(255LL<<(PATWC*8));
+	v |= 1LL<<(PATWC*8);	/* WC */
+	wrmsr(0x277, v);
 }
 
 /*
@@ -535,3 +544,28 @@ vunmap(void *v, int)
 {
 	paddr(v);	/* will panic on error */
 }
+
+/*
+ * mark pages as write combining (used for framebuffer)
+ */
+void
+patwc(void *v, int n)
+{
+	uintptr *pte, mask, attr, va;
+	int z, l;
+
+	/* set the bits for all pages in range */
+	for(va = (uintptr)v; n > 0; n -= z, va += z){
+		l = 0;
+		pte = mmuwalk(m->pml4, va, l, 0);
+		if(pte == 0)
+			pte = mmuwalk(m->pml4, va, ++l, 0);
+		if(pte == 0 || (*pte & PTEVALID) == 0)
+			panic("patwc: va=%#p", va);
+		z = PGLSZ(l);
+		z -= va & (z-1);
+		mask = l == 0 ? 3<<3 | 1<<7 : 3<<3 | 1<<12;
+		attr = (((PATWC&3)<<3) | ((PATWC&4)<<5) | ((PATWC&4)<<10));
+		*pte = (*pte & ~mask) | (attr & mask);
+	}
+}