On Mon, Feb 27, 2012 at 04:27:23PM +0000, Ian Campbell wrote: > On Thu, 2012-02-23 at 17:48 +0000, Stefano Stabellini wrote: > > We need a register to pass the hypercall number because we might not > > know it at compile time and HVC only takes an immediate argument. > > > > Among the available registers r12 seems to be the best choice because it > > is defined as "intra-procedure call scratch register". > > R12 is not accessible from the 16 bit "T1" Thumb encoding of mov > immediate (which can only target r0..r7). This is untrue. The important instructions, like MOV Rd, Rn can access all the regs. But anyway, there is no such thing as a Thumb-1 kernel, so we won't really care. > Since we support only ARMv7+ there are "T2" and "T3" encodings available > which do allow direct mov of an immediate into R12, but are 32 bit Thumb > instructions. > > Should we use r7 instead to maximise instruction density for Thumb code? The difference seems trivial when put into context, even if you code a special Thumb version of the code to maximise density (the Thumb-2 code which gets built from assembler in the kernel is very suboptimal in size, but there simply isn't a high proportion of asm code in the kernel anyway.) I wouldn't consider the ARM/Thumb differences as an important factor when deciding on a register. One argument for _not_ using r12 for this purpose is that it is then harder to put a generic "HVC" function (analogous to the "syscall" syscall) out-of-line, since r12 could get destroyed by the call. If you don't think you will ever care about putting HVC out of line though, it may not matter. Cheers ---Dave -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
