Re: [RFC] MIPS: Install final length of TLB refill handler rather than 256 bytes

[Fredrik Noring <noring@nocrew.org>]

Hi Phil,

> There is a comment about the R4000 worst case:
> 
>  /* The worst case length of the handler is around 18 instructions for
>   * R3000-style TLBs and up to 63 instructions for R4000-style TLBs.
>   * Maximum space available is 32 instructions for R3000 and 64
>   * instructions for R4000.
> 
> So you need to check the handler generated for your cpu doesn't exceed
> your 32 instructions.

Do you mean like this:

> On 4/5/19 6:05 PM, Fredrik Noring wrote:
> > I have a separate patch that checks the R5900 handler length limit,
> > but it depends on R5900 support, which isn't merged (yet).

I have now attached this separate patch for reference, see below.

> Maybe you could modify the logic few lines earlier that check and
> panic("TLB refill handler space exceeded") and add a case for your cpu
> type. There you could set a handler_max_size = 0x80, 0x100 else.
> 
> Take my comment as RFC too, I'm just wondering :)

To check the length we would need to define CPU_R5900 first. :)

Are any MIPS kernel maintainers happy to review an initial R5900 patch
submission?

[ The patch in the original RFC is generic and it does not depend on
the availability of CPU_R5900, although it avoids clobbering the R5900
performance counter handler. ]

Fredrik

--- a/arch/mips/mm/tlbex.c
+++ b/arch/mips/mm/tlbex.c
@@ -1399,6 +1399,10 @@ static void build_r4000_tlb_refill_handler(void)
 	 * unused.
 	 */
 	switch (boot_cpu_type()) {
+	case CPU_R5900:
+		if ((p - tlb_handler) > 32)
+			panic("TLB refill handler space exceeded");
+		/* Fallthrough */
 	default:
 		if (sizeof(long) == 4) {
 	case CPU_LOONGSON2: