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Re: [PATCH v2] Virtual memory size detection for 64 bit MIPS CPUs

Subject: Re: [PATCH v2] Virtual memory size detection for 64 bit MIPS CPUs
From: David Daney <>
Date: Mon, 01 Feb 2010 14:12:40 -0800
Cc: "" <>
In-reply-to: <1265061872.5825.71.camel@groeck-laptop>
Original-recipient: rfc822;
References: <> <> <1265061872.5825.71.camel@groeck-laptop>
User-agent: Thunderbird (X11/20090320)
Guenter Roeck wrote:
On Mon, 2010-02-01 at 16:42 -0500, David Daney wrote:
Guenter Roeck wrote:
+static inline void cpu_set_vmbits(struct cpuinfo_mips *c)
+       if (cpu_has_64bits) {
+               unsigned long zbits;
+               asm volatile(".set mips64\n"
+                            "and %0, 0\n"
+                            "dsubu %0, 1\n"
+                            "dmtc0 %0, $10, 0\n"
+                            "dmfc0 %0, $10, 0\n"
+                            "dsll %0, %0, 2\n"
+                            "dsra %0, %0, 2\n"
+                            "dclz %0, %0\n"
+                            ".set mips0\n"
+                            : "=r" (zbits));
+               c->vmbits = 64 - zbits;
+       } else
+               c->vmbits = 32;
It should be possible to express this in 'pure' C using read_c0_entryhi()/write_c0_entryhi(), also you need to be sure you are

Sure, no problem.

not writing 1s to any reserved bits of the register.

That may be tricky, since the upper bits are reserved in some
architectures. For example, the 20Kc core specification says that bits
61:40 are reserved and must be written with 0.

I can write, say, 0x3fffffffffff0000 to avoid writing into lower
reserved bits, but that won't help for any upper reserved bits. Would
that be acceptable / better ?

The MIPS64® Privileged Resource Architecture manual might be a better reference.

I would set 1s only in bits 13-63 (the VPN2 and R fields). We don't support and don't really care about VPN2X, the upper bits are really the only ones of interest here.

David Daney

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