Hi Richard, On 27/05/14 03:38, Richard Henderson wrote: > On 05/26/2014 07:32 PM, Petar Jovanovic wrote: >> >> ________________________________________ >> From: Richard Henderson [rth7...@gmail.com] on behalf of Richard Henderson >> [r...@twiddle.net] >> Sent: Tuesday, May 27, 2014 3:35 AM >> To: Petar Jovanovic; Petar Jovanovic; qemu-devel@nongnu.org >> Cc: aurel...@aurel32.net; James Hogan >> Subject: Re: [v2 PATCH] target-mips: implement UserLocal Register >> >> On 05/26/2014 06:21 PM, Petar Jovanovic wrote: >>> Because we do not know the content of CP0_HWREna. >> >>> We can know the same way we know the content of CP0_Config3. >> >> Almost all the bits in CP0_Config3 including ULRI bit are read-only and >> preset by >> hardware, thus checking bit ULRI at translation time is the same as checking >> it >> at execution time. How can we know the content of CP0_HWREna at translation >> time? > > That's what tb->flags and env->hflags. An encoding of cpu state at > translation > time.
I agree for the Config3 bit that it shouldn't change during execution of the guest so can be done at translation time, but I'm not convinced it's worth wasting tb->flags bits for CP0_HWREna state which the guest OS is free to change at any time, just to save a single branch each time it's used (which could be done in tcg easily enough to avoid the helper). Only a minority of code uses RDHWR, although admittedly UserLocal is the most widespread of the RDHWR registers due to it's use for thread local storage. A quick and non-scientific look (assuming all code is equal) at my MIPS32 buildroot userland shows 1286 rdhwrs of UserLocal in 14 million total lines of objdump (1 in 10929 instructions), and only a handful of the other rdhwr registers. Cheers James
