MIPS: MemoryMapID (MMID) Support

Introduce support for using MemoryMapIDs (MMIDs) as an alternative to
Address Space IDs (ASIDs). The major difference between the two is that
MMIDs are global - ie. an MMID uniquely identifies an address space
across all coherent CPUs. In contrast ASIDs are non-global per-CPU IDs,
wherein each address space is allocated a separate ASID for each CPU
upon which it is used. This global namespace allows a new GINVT
instruction be used to globally invalidate TLB entries associated with a
particular MMID across all coherent CPUs in the system, removing the
need for IPIs to invalidate entries with separate ASIDs on each CPU.

The allocation scheme used here is largely borrowed from arm64 (see
arch/arm64/mm/context.c). In essence we maintain a bitmap to track
available MMIDs, and MMIDs in active use at the time of a rollover to a
new MMID version are preserved in the new version. The allocation scheme
requires efficient 64 bit atomics in order to perform reasonably, so
this support depends upon CONFIG_GENERIC_ATOMIC64=n (ie. currently it
will only be included in MIPS64 kernels).

The first, and currently only, available CPU with support for MMIDs is
the MIPS I6500. This CPU supports 16 bit MMIDs, and so for now we cap
our MMIDs to 16 bits wide in order to prevent the bitmap growing to
absurd sizes if any future CPU does implement 32 bit MMIDs as the
architecture manuals suggest is recommended.

When MMIDs are in use we also make use of GINVT instruction which is
available due to the global nature of MMIDs. By executing a sequence of
GINVT & SYNC 0x14 instructions we can avoid the overhead of an IPI to
each remote CPU in many cases. One complication is that GINVT will
invalidate wired entries (in all cases apart from type 0, which targets
the entire TLB). In order to avoid GINVT invalidating any wired TLB
entries we set up, we make sure to create those entries using a reserved
MMID (0) that we never associate with any address space.

Also of note is that KVM will require further work in order to support
MMIDs & GINVT, since KVM is involved in allocating IDs for guests & in
configuring the MMU. That work is not part of this patch, so for now
when MMIDs are in use KVM is disabled.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org

1 files changed, 41 insertions, 11 deletions

diff --git a/arch/mips/mm/tlb-r4k.c b/arch/mips/mm/tlb-r4k.c
index 0114c43398f3..c13e46ced425 100644
--- a/arch/mips/mm/tlb-r4k.c
+++ b/arch/mips/mm/tlb-r4k.c
@@ -120,14 +120,23 @@ void local_flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 		if (size <= (current_cpu_data.tlbsizeftlbsets ?
 			     current_cpu_data.tlbsize / 8 :
 			     current_cpu_data.tlbsize / 2)) {
-			int oldpid = read_c0_entryhi();
+			unsigned long old_entryhi, uninitialized_var(old_mmid);
 			int newpid = cpu_asid(cpu, mm);
 
+			old_entryhi = read_c0_entryhi();
+			if (cpu_has_mmid) {
+				old_mmid = read_c0_memorymapid();
+				write_c0_memorymapid(newpid);
+			}
+
 			htw_stop();
 			while (start < end) {
 				int idx;
 
-				write_c0_entryhi(start | newpid);
+				if (cpu_has_mmid)
+					write_c0_entryhi(start);
+				else
+					write_c0_entryhi(start | newpid);
 				start += (PAGE_SIZE << 1);
 				mtc0_tlbw_hazard();
 				tlb_probe();
@@ -143,7 +152,9 @@ void local_flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 				tlb_write_indexed();
 			}
 			tlbw_use_hazard();
-			write_c0_entryhi(oldpid);
+			write_c0_entryhi(old_entryhi);
+			if (cpu_has_mmid)
+				write_c0_memorymapid(old_mmid);
 			htw_start();
 		} else {
 			drop_mmu_context(mm);
@@ -203,15 +214,21 @@ void local_flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
 	int cpu = smp_processor_id();
 
 	if (cpu_context(cpu, vma->vm_mm) != 0) {
-		unsigned long flags;
-		int oldpid, newpid, idx;
+		unsigned long uninitialized_var(old_mmid);
+		unsigned long flags, old_entryhi;
+		int idx;
 
-		newpid = cpu_asid(cpu, vma->vm_mm);
 		page &= (PAGE_MASK << 1);
 		local_irq_save(flags);
-		oldpid = read_c0_entryhi();
+		old_entryhi = read_c0_entryhi();
 		htw_stop();
-		write_c0_entryhi(page | newpid);
+		if (cpu_has_mmid) {
+			old_mmid = read_c0_memorymapid();
+			write_c0_entryhi(page);
+			write_c0_memorymapid(cpu_asid(cpu, vma->vm_mm));
+		} else {
+			write_c0_entryhi(page | cpu_asid(cpu, vma->vm_mm));
+		}
 		mtc0_tlbw_hazard();
 		tlb_probe();
 		tlb_probe_hazard();
@@ -227,7 +244,9 @@ void local_flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
 		tlbw_use_hazard();
 
 	finish:
-		write_c0_entryhi(oldpid);
+		write_c0_entryhi(old_entryhi);
+		if (cpu_has_mmid)
+			write_c0_memorymapid(old_mmid);
 		htw_start();
 		flush_micro_tlb_vm(vma);
 		local_irq_restore(flags);
@@ -290,9 +309,13 @@ void __update_tlb(struct vm_area_struct * vma, unsigned long address, pte_t pte)
 	local_irq_save(flags);
 
 	htw_stop();
-	pid = read_c0_entryhi() & cpu_asid_mask(&current_cpu_data);
 	address &= (PAGE_MASK << 1);
-	write_c0_entryhi(address | pid);
+	if (cpu_has_mmid) {
+		write_c0_entryhi(address);
+	} else {
+		pid = read_c0_entryhi() & cpu_asid_mask(&current_cpu_data);
+		write_c0_entryhi(address | pid);
+	}
 	pgdp = pgd_offset(vma->vm_mm, address);
 	mtc0_tlbw_hazard();
 	tlb_probe();
@@ -358,12 +381,17 @@ void add_wired_entry(unsigned long entrylo0, unsigned long entrylo1,
 #ifdef CONFIG_XPA
 	panic("Broken for XPA kernels");
 #else
+	unsigned int uninitialized_var(old_mmid);
 	unsigned long flags;
 	unsigned long wired;
 	unsigned long old_pagemask;
 	unsigned long old_ctx;
 
 	local_irq_save(flags);
+	if (cpu_has_mmid) {
+		old_mmid = read_c0_memorymapid();
+		write_c0_memorymapid(MMID_KERNEL_WIRED);
+	}
 	/* Save old context and create impossible VPN2 value */
 	old_ctx = read_c0_entryhi();
 	htw_stop();
@@ -381,6 +409,8 @@ void add_wired_entry(unsigned long entrylo0, unsigned long entrylo1,
 	tlbw_use_hazard();
 
 	write_c0_entryhi(old_ctx);
+	if (cpu_has_mmid)
+		write_c0_memorymapid(old_mmid);
 	tlbw_use_hazard();	/* What is the hazard here? */
 	htw_start();
 	write_c0_pagemask(old_pagemask);