Messages in this thread | | | Subject | Re: [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor | From | Ian Campbell <> | Date | Tue, 28 Feb 2012 10:12:50 +0000 |
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On Mon, 2012-02-27 at 21:05 +0000, Peter Maydell wrote: > On 27 February 2012 16:27, Ian Campbell <Ian.Campbell@citrix.com> wrote: > > R12 is not accessible from the 16 bit "T1" Thumb encoding of mov > > immediate (which can only target r0..r7). > > > > 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? > > r7 is (used by gcc as) the Thumb frame pointer; I don't know if this > makes it worth avoiding in this context.
I think it does.
It actually sounds as if using r12 is fine here, the impact on code density should be pretty small -- there aren't really all that many call sites which involve hypercalls.
By way of an example I measured an x86 kernel which should be using more hypercalls due to pv paging etc and found that 0.014% of the lines in "objdump -d" contained a call to the hypercall_page. (I know not all lines of objdump -d output are instructions but it's a reasonable approx IMHO).
So I think using 3 16 bit instructions slots instead of 2 won't make much impact in practice.
Thanks, Ian.
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